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
I still think it's better to double check the type, then having random runtime panics when someone inits the storage with some wrong repository type.