Add IETF RFC 7009 Token Revocation

• Added IETF RFC 7009 Token Revocation
  • OAuth2::Client#revoke_token
  • OAuth2::AccessToken#revoke
• OAuth2::AccessToken#refresh now supports block param pass through
• OAuth2.config is no longer writable
  • the hash it contains still is, of course!
• Errors raised by OAuth2::AccessToken are now always OAuth2::Error and have better metadata
• Fixes https://github.com/oauth-xx/oauth2/issues/224, #224 (closed)