Use common header for remaining web views (source code, disassembly, peek).

Details:
* Made it easier to reuse css, javascript, etc. across views.
* Added two new templates: sourcelisting, plaintext.
* Use new templates for showing source code, disassembly, peek.
* Merged Functions menu into View menu.
* View change menu entries are now always enabled.
* Removed now unused "newWindow" argument to navigate.
* Removed redundancy in web handlers.
* Renamed /weblist URL to /source.
* Assembly and source code listings are now limited to 500 functions
  sorted by decreasing flat value (to prevent huge delays when
  selecting asm/source view across the entire profile).

* Split template into multiple defined templates for ease of sharing.

* Add a menu bar at the top and tie all buttons to it.

* Add top view.

* Handle top table clicks.

* Handle regexp based selection of top entries.

* Added test for /top view.

* Reduced code duplication between / and /top handlers.

* Add Refine menu to Top view as well.

Details:
* Pass base URL to web page.
* Use base URL in navigate() to stay in same view.
* Change Reset to stay in same view.

* Handled review comments + increased top display from 50 to 500 rows:

* Moved "inline" indicator to separate column of top display.
* Moved "Reset" button to "Refine" menu.
* Simplified some Javascript.
* Handle all meta characters in quotemeta.

* Removed unnecessary css "display: inline" from closedetails.

Signed-off-by: Sanjay Ghemawat <sanjay@alum.mit.edu>

When wrap is nil but isLocal is false, we segfault when we attempt to wrap mux handlers.

when visiting the webui.

When net.SplitHostPort is called, it will return the string "::1",
but the code was only checking for "[::1]".

Detailed list of changes:
* Since the list of actions is now long, move it into
  an action box that is displayed only when applicable.
* Display help messages when hovering over action buttons.
* Hitting return in the search box invokes "focus" action.
* Cleaned up stale css entries.

I have attached one of the resulting pages to see what things look like easily.  It is static, so won't allow navigation to other pages, but should be interactive enough.

[benchcpu.zip](https://github.com/google/pprof/files/1151238/benchcpu.zip)

* Allow -http to accept host:port

* Add serveWebInterface tests.

* Make sure serveWebInterface is serving on the URL.

* Avoid nil pointer dereference

* Simplify the -http flag test.

* Use the right capitalization for hostport.

* Fix the double escaping in regexps.

* Address review comments.

* 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.