First, horrendously bad, cut at trying to use JGit for realz in SBT.  If we can succeed in the long run, it would be a huge improvement for cross-platform git users.
Improve JGit capabilities.

* Bump version to 0.6 to denote major change.
* Add `git-head-commit`  to detect latest git commit version
* Add `git-top-tag` key to detect if we're on a tagged commit.
* Implement hacky methods of detection in both JGit and console.

One step closer to JGit by default!