Examples

This PR contains ports of some of the [libgit2 examples](https://github.com/libgit2/libgit2/tree/development/examples) to Ruby/Rugged.

* [x] Blame
* [x] Diff
* [x] Log
* [ ] ...

This helped me in fixing some bugs in Rugged and finding functionality that was not added to the bindings, and should help newcomers on getting a grip on how to perform basic git actions through Rugged.

I'll add more as soon as I port them over.