Move from Grit to Rugged for git processing

This pull request addresses #60 as an attempt to move from [mojombo/grit](https://github.com/mojombo/grit) to [libgit2/rugged](https://github.com/libgit2/rugged).

Large change set here! Difficult to split this up into modular changes as Grit at the time was the heart of the processing. The initial implementation done here for Rugged works, and all the specs are currently passing.

The GPG signing issue is no longer an issue, and Rugged also provides a substantial speed boost and memory reduction than the Grit implementation. The current state also run on Ruby 2.0.x as we are not held back by Grit.