(proposal): use registry-to-registry transfer

Instead of running a docker engine in the controller, I implemented a new feature to the registry that allows a user to pull in a repository from one registry to another. This greatly simplifies the `deis-build` PR. Note that a couple of preconditions must be met with `deis-build` now:

 - you **must** supply a URL scheme (such as `deis build http://example.com:5000/example`
 - the public index does not work due to the endpoints being closed source. This must be implemented upstream in https://github.com/deis/docker-registry/compare/repository-import.

The meat of this is hard to write tests for, but will get added eventually. I just needed this PR to work and get some discussion going :)