Changed node-gitteh to nodegit

Current, node-gitteh doesn't have any active maintainers and hasn't had any major updates in a while. NodeGit has just had a major update where we updated to the latest libgit2 release (0.21.2) and also has 3 active maintainers (me, @tbranyen, and @maxkorp).

I think that this would help people use libgit2 inside of nodejs with a more active library and more libgit2 functionality than they currently have.

Thoughts?