Remove dependencies on registry packages

Because docker core cannot vendor non-master Go dependencies, we need to remove
dependencies on registry package. The definition of digest.Digest has been
changed to a string and the regular expressions have been ported from
docker-registry/common library.

We'll likely change this be dependent on the registry in the future when the
API stabilizies and use of the master branch becomes the norm.

Signed-off-by: Stephen J Day <stephen.day@docker.com>