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