implement git deploy method

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
add git deployment into deployment list

Adds Git to the deployment list in deployment.go to actually be used by drone.
Merge pull request #52 from antonlindstrom/feature-deploy-git

Implement feature git deploy