Update gen_release to use git archive instead of svn export.
Use working copy as repo url when testing release tarball.
Support building branches with gen_release.
Merge pull request #409 from thomasvandoren/migrate-gen-release-to-git

Migrate gen_release to git.

Use git archive instead of svn export in gen_release. Updates the automated test script that calls
gen_release to call git archive on the working copy of the repo instead of going to github (providing a
bit of a perf advantage).