Added support for gems on git repositories

Hey @cyx,

How're you doing?

I've added support for gems on git repositories - ref. #9.

I know this adds a good number of lines to the codebase and the aim is to keep it simple. Anyway, it might be a good starting point if this feature is of interest.

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.


Let me know what you think about it :)
Thanks!