==================
- Refacotor oauth.Client to take id/secret as constructor args.
- Add basic caching on oauth token verification.
- Accept option "/v1" suffix on server URLs.
- Add get_identity_assertion() method to core.Session.
- Add methods to oauth.Client for authorizing codes and tokens.
- Add a new error hierarchy for trust-related errors.
- Additional sanity-checking in oauth scope checks.