Added support for git-based repositories.
You can now:

`dep add gs --git git://github.com/soveran/gs`

and it will pull `gs` from its git repository.

If you happen to have many gems on one repository, you can also do:

`dep add padrino-performance --git
git://github.com/padrino/padrino-framework padrino-performance`

Doing an install will clone the repo in a temporary directory, build
and install the gem into your gemset.