Replace Grit with Rugged

Implement all classes using Rugged objects instead of Grit
Update README

Use the rugged object instead of grit in the README's examples.
Update SimpleCov dependency

The previously-used version of SimpleCov doesn't seem to play well with
the forking that Repository does when archiving a repo to a file.
Remove Grit dependency

Remove Grit from the gemspec, and remove the ability to wrap Grit
objects in the Commit and Diff classes.