Merge pull request #182 from vito/git-deploy-use-branch

git deploy pushes to given branch (default master)
support for git branch promotion strategies

This allows drone to be used as a pseudo-git-pipeline; the deploy step
specifies branches to promote from and to. For example, `master: passed`
instructs the deploy step to promote to `passed` if we're the `master` branch
and just went green.

Signed-off-by: David Varvel <dvarvel@pivotallabs.com>
Merge branch 'multi-git-branch' into goodies