Add support for images & image repositories

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
Agreed but I think it's necessary. I'm also wondering how overrides should work. Should they always replace? Is there a case where they should be additive (e.g. if you specify an env var for maxheap=2gb, but the image has other env vars, should you be required to specify all of them or just the ones you want to change)?