pygit2 and colors

* Uses pygit2 if it's installed otherwise it falls back to git-python.
* Colored output for the rainbow and unicorn lovers via colorama.

![colorama](https://cloud.githubusercontent.com/assets/1388/3717716/8558ab82-162a-11e4-8d28-846c5fb5b201.png)
Can you add it also to `INSTALL.rst`. The important message is to have same version of `libgit2` and `pygit2`!
**Mac OS**
```console
$ brew update
$ brew install libgit2
$ pip install pygit2
```