WIP - OAuth plugin

Initial work on an OAuth plugin package, based on github.com/openshift/origin/tree/master/pkg/oauth

- [X] API objects to back OAuth server implementation
- [ ] Rework etcd registry to use new generic registry
- [ ] Token authenticator implementation which validates tokens as unexpired OAuth tokens
- [ ] Pull in github.com/RangelReale/osin as godep
- [ ] Add interfaces for pluggable login, challenge, and grant flows
- [ ] Define osin-based OAuth server
- [ ] Add a plugin/cmd/oauth command
These belong in `register.go` now