Set base address on OSX.

This fixes issue #311. The mapped address of libraries was never considered, nor was the load address of the segments. It also fixes a couple of issues in binutils.go, such as when the tail of the data wasn't handled right when grouping symbols at the same address.

* Make binutils thread safe.

binutils is now used from multiple goroutines when the
web interface is enabled (since http server spins up a new
goroutine for each request). Added appropriate synchronization
to various objects in binutils to provide the appropriate
thread-safety.

In addition, added a bunch of tests:

. Test parsing of nm output.
. Tests for Binutils methods.
. Test llvm-symbolizer interface (using an llvm-symbolizer emulator).

* Fix go vet errors about unkeyed literals.

* copy stderr to figure out test failures

* Make shell script portable.

* Update comments to match new var name.

* Give a name to the embedded Mutex field in addr2Liner.