R1:0327a5fc69ae

OAuthServer polish and random sauce

Summary:
This diff makes the OAuthServer more compliant with the spec by
- making it return well-formatted error codes with error types from the spec.
- making it respect the "state" variable, which is a transparent variable the
client passes and the server passes back
- making it be super, duper compliant with respect to redirect uris
-- if specified in authorization step, check if its valid relative to the client
registered URI and if so save it
-- if specified in…