Add support for federating to Cloud Foundry UAA

[ericchiang]

Add a connector for logging in with Cloud Foundry UAA. Will probably look something like the current bitbucket connector[0] since we've been told that UAA supports some flavor of OAuth2.

cc @jbowen93 @whitlockjc

[0] https://github.com/coreos/dex/blob/master/connector/connector_bitbucket.go

[whitlockjc]

I'll get cracking on this.