make git version available to the build process

If the user sets config var GO_GIT_DESCRIBE_SYMBOL, we
add a linker flag to assign the output of 'git describe'
to the specified symbol.

closes #50
use commit id for version symbol if no tag exists

This makes GO_GIT_DESCRIBE_SYMBOL useful even if no tags
have been pushed to Heroku.
omit needless words
Update virtualenv to 1.11.6

1.7 doesn't appear to play nicely with the version of Python installed
on Ubuntu's trusty.
Only do the symlink if envdir is passed in and you're setting this stuff
up

As it is now we can't use this buildpack in non dyno environments w/o
/app
idempotent symlink

The official interface doc for buildpacks[1] doesn't say
that buildpacks always run in a fresh container or
anything of the like, so we ought to assume that things
we did in previous invocations might stick around.

[1]: https://devcenter.heroku.com/articles/buildpack-api