Add a custom type/provider to manage homebrew

This allows us to require a minimum revision of homebrew, like so:

```puppet
homebrew_repo { '/opt/boxen/homebrew':
  min_revision => 'd34db33f',
}
```
Require a minimum homebrew revision

This revision is one that provides some functionality that
puppet-homebrew is going to need soon, so I chose it as the first rev to
depend on.
Raise unless we're running in Boxen
You can't test for resources with underscores :(
Remove debug output
Merge remote-tracking branch 'refs/remotes/origin/master' into manage-homebrew-revision

Conflicts:
	manifests/repo.pp
Merge pull request #42 from boxen/manage-homebrew-revision

Manage homebrew revision