Make bin/build self-contained

This commit makes the build script usable for distributing the
executable, e.g. from a Homebrew formula.

- It works from any directory, including outside of the GOPATH
- Dropped the dependency on godep and hence hg

Signed-off-by: Max Brunsfeld <mbrunsfeld@pivotallabs.com>
Merge pull request #175 from cloudfoundry/self-contained-build-script

Make bin/build self-contained