Add "trim path" option which can be used to relocate sources.

When pprof is asked to show annotated source for a profile collected on
another machine for a Go program, the profile contains absolute paths
which may not exist on the local machine. In that case pprof currently
fails to locate the source with no option to help it. The new option
adds a way to specify one or several source path prefixes that should be
trimmed from the source paths in the profile before applying the search
using search path option.

For example, taking the example from the issue where the source file
path in the profile is
/home/teamcitycpp/agent09/work/56cbaf9067/_gopath/src/badoo/lakafka/main.go
and the local path is /home/marko/lakafka/main.go. The user may
specify
`-trim-path=/home/teamcitycpp/agent09/work/56cbaf9067/_gopath/src/badoo`
to make pprof find the source. The source path doesn't need to be
specified if the current working dir is anything at or under
`/home/marko/`.

When the trim path is not specified, it is guessed heuristically based
on the basename of configured searched paths. In the example above,
setting `-search-path=/home/marko/lakafka` would be sufficient to
activate the heuristic successfully. Or having the current directory as
`/home/marko/lakafka` since the search path is by default set to the
current working directory. Note that the heuristic currently
does not attempt to walk the configured search paths up like the search
does. This is to keep it simple, use `-trim-path` explicitly in more
complicated cases.

Fixes #262.

The third_party svg directory has been a mix of pprof-specific code and
svgpan itself. This change moves the pprof-specific code out and adds a
license file. It also updates svgpan to 1.2.2 from
https://github.com/aleofreddi/svgpan while we are here. It also renames
the directory to svgpan to have all directories under third_party be
named precisely over the third party package they hold.

* Modified variable type to have []string value

* added repeatableStringKind

* modify repeatableStringKind to actually handle multiple values

* Modify applyFocus to work with multiple values for tagfocus/tagignore

* Modify applyFocus to work with multiple values for tagfocus/tagignore

* Modified CompileTagFilter to handle tag filters of the form -tagfocus=key1=val1

* Modified StringList

* add testing for compileTagFilter

* fixed typo

* fixed code style error

* Revert "fixed code style error"

This reverts commit 0b1826c163.

* Revert "fixed typo"

This reverts commit 2593652bd0.

* Revert "add testing for compileTagFilter"

This reverts commit 152d7b0363.

* Revert "Modified StringList"

This reverts commit b57d35a762.

* Revert "Modified CompileTagFilter to handle tag filters of the form -tagfocus=key1=val1"

This reverts commit ebe1e344bf.

* Revert "Modify applyFocus to work with multiple values for tagfocus/tagignore"

This reverts commit 3b62091b67.

* Revert "Modify applyFocus to work with multiple values for tagfocus/tagignore"

This reverts commit 7172159cd3.

* Revert "modify repeatableStringKind to actually handle multiple values"

This reverts commit abb1e3ec4b.

* Revert "added repeatableStringKind"

This reverts commit 8b8e4e76fc.

* Revert "Modified variable type to have []string value"

This reverts commit 5f213dc4d1.

* added support for filtering by specific tags

* Fixed formatting problems

* Updated variable name for clarity

* Added test cases for tag filtering

* Update tagfocus and tagignore description

* Fixed formatting error

* Updated docs to explain new tag filtering

* Updated docs to clarify tag filtering

* Updated docs to clarify tag filtering

* Updated docs and help comments for tag filtering

* Updated docs for tag filtering

* Update pprof docs for tag filtering

* Update tagfocus and tagignore help descriptions

* Update tagfocus and tagignore descriptions in docs

* Update tagfocus and tagignore descriptions in docs and help

* Updated tagfocus and tagignore descriptions in docs; added TODO for regexp matching

* Updated tagfocus and tagignore descriptions in docs

* Modified docs fo tagfocus for clarity

* Added an interactive web interface triggered by passing -http=port
on the command line.  The interface is available by visiting
localhost:port in a browser.

Requirements:
* Graphviz must be installed.
* Browser must support Javascript.
* Tested in recent stable versions of chrome and firefox.

Features:
* The entry point is a dot graph display (equivalent to "web" output).
* Nodes in the graph can be selected by clicking.
* A regular expression can also be typed in for selection.
* The current selection (either list of nodes or a regexp)
  can be focused, ignored, or hidden.
* Source code or disassembly of the current selection can be displayed.

* Remove unused function.

* Skip graph generation test if graphviz is not installed.

* Added -http port and the various modes of using pprof to the
usage message.

* Web interface now supports "show" option.

* Web interface automatically opens the browser pointed at
the page corresponding to command line arguments.

* Some tweaks for firefox.

* Handle review comments (better usage message, more testing).

* Handled review comments:

1. Capture and display errors like "Focus expression matched no samples".
2. Re-ordered buttons to match other interfaces.
3. Use UI.PrintErr to print error messages.

* Handle javascript code review comments (a bunch of cleanups).
Also added pprof binary to .gitignore.

* `git ls-files -- '*.go' | xargs gofmt -s -w`

* driver: unexport InternalOptions

This was an exported method that returned an internal type, so it's
unlikely that anyone depends on its presence.

* internal/binutils: remove newline in error string

golint reported: error strings should not be capitalized or end with
punctuation or a newline

* internal/driver: s/buildId/buildID/ per golint

* internal/elfexec: remove punctuation in error string

golint reported: error strings should not be capitalized or end with
punctuation or a newline

* internal/graph: correct comment

Found with golint.

* internal/graph: add method comment

golint reported: exported method Edge.WeightValue should have comment
or be unexported

* internal/report: remove newline in error string

golint reported: error strings should not be capitalized or end with
punctuation or a newline

* *: remove unused code

Found with honnef.co/go/tools/cmd/unused:

	internal/binutils/disasm_test.go:137:8: const objdump is unused (U1000)
	internal/driver/driver_test.go:353:6: type testProfile is unused (U1000)
	internal/graph/graph.go:838:6: func countEdges is unused (U1000)
	profile/proto.go:148:6: func encodeStringOpt is unused (U1000)

* *: simply code

Found with honnef.co/go/tools/cmd/gosimple:

	internal/driver/commands.go:471:24: should omit comparison to bool constant, can be simplified to !b (S1002)
	internal/driver/driver.go:163:5: should omit comparison to bool constant, can be simplified to !trim (S1002)
	internal/driver/driver.go:168:5: should omit comparison to bool constant, can be simplified to !focus (S1002)
	internal/driver/driver.go:172:5: should omit comparison to bool constant, can be simplified to !tagfocus (S1002)
	internal/driver/driver.go:176:5: should omit comparison to bool constant, can be simplified to !hide (S1002)
	internal/graph/dotgraph.go:213:34: should use make(map[string][]*Tag) instead (S1019)
	internal/report/report.go:1061:3: should replace loop with label = append(label, rpt.options.ProfileLabels...) (S1011)
	profile/proto.go:157:5: should omit comparison to bool constant, can be simplified to !x (S1002)

* *: correct mistakes

Found with honnef.co/go/tools/cmd/staticcheck:

	driver/driver.go:280:20: infinite recursive call (SA5007)
	internal/driver/driver_focus.go:54:11: this value of err is never used (SA4006)
	profile/proto.go:74:32: x | 0 always equals x (SA4016)

* Add linters to .travis.yml

Previously we were showing filenames together with function names
when generating reports at function granularity. This was an attempt
to provide more information to the user, but it clutters the output.
Also, in cases where a function is associated to multiple files
(eg template instantiations in C++), keeping the filename may prevent
us from combining nodes from the same function.

Will stop keeping filenames at the default function granularity, and will
remove the "functionnameonly" option which was previously providing this
functionality.

For certain formats (eog, evince, gv, web, weblist), the visualizer
would be invoked regardless of whether the user specified a specific
output file to write to. In order to ensure that output is properly
written, the invokeVisualizer function has a check for whether the
output is os.Stdout and alters behavior based on that. This is the
wrong abstraction layer to do that work.

At the core of the problem is that PostProcess is used to both
do data post-processing that is inherent to the format, and also
to invoke some visualization for the data (to Stdout or browser).
We split these two steps apart and make it obvious which is which
by adding a visualizer field to Command.

This seperation of concerns allows us to simplify the code.

By default hide entries in the comment field that starts with #, and
add a "comments" command to show all comments.

Only create $HOME/pprof if a profile is stored remotely
and must be stored. Also, create non-profile temps on
os.TempDir() instead of $HOME/pprof or /tmp.

Rather than print options on each prompt, add a "options" command
that prints the current options in a user friendly format. Also,
make sure that those options can be parsed back as printed.

This enables pprof extensions to update the default for a variable
before the command line is processed.

Currently pprof will look for source files only on the current directory
and its parents. This makes it hard to examine sources on jobs where
there are multiple source trees (eg from different libraries).

Add a variable to provide a search path for source files. It will default
to the cwd, so there will be no change in behavior by default.

This reverts commit 9c191ebbce.
The trimproto option is no longer needed, removing it to avoid code growth.

Add new trimproto option that generates a new profile while removing
symbol information for functions below nodefraction. This reduces the
profile sizes significantly.

Add field for profile sources to indicate what is the primary
sample value, which visualizers should display by default.
Previously there was a convention for pprof to use the last
sample_index by default, this provides more flexibility.

Also update the build instructions to use go get