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
Remove unneeded godep from build

As described in this GitHub comment[1] the use of godep in the build
script is unneeded.  Having it there requires all builders of the
project to have it installed even though it never needs to be called.
As an example, this causes the Homebrew formula for the project to be
massively more complex than it needs to be.  This change removes that
use of godep in the buildscript, instead calculating the absolute path
to the checked-in godep workspace and using that.

[1]: https://github.com/XenoPhex/homebrew/commit/a885c669908886d4cf9214ed1e812bd29dc7e217#commitcomment-6494522