Implement feature git deploy

This commit implements the git deploy method. This is basically a copy of the
Heroku deploy method except it uses a target instead of an app. The target can
be anything that git accepts as a url.

An example of deploy using git can be:

    deploy:
      git:
        target: git@example.com:myapp.git

Would be awesome if a second pair of eyes could look at the code so it looks alright.

Thanks!