Add support for images & image repositories

The first commit will likely go away assuming @csrwng's builds PR lands first. (#43)

This adds support for images and image repositories to origin. It also adds a webhook for the Docker registry that can be invoked whenever a new tag is pushed to a Docker image repository. The webhook receives data from the registry (repository, image id, image metadata, tag) and stores the data in origin. The corresponding Docker registry changes are here: https://github.com/ncdc/docker-registry/compare/openshift_hook

**TODO**
- [ ] godoc comments
- [ ] tests
- [x] etcd registry implementation
- [ ] validation methods
- [ ] kubecfg support
In create it's ok not to (it's inconsistent upstream) because the only person who should be passing things to Create are the RESTHandlers which by contract only pass the result of New()