Initialize OAuth client from registry.

def client(self, client_name, **params):
        """Initialize OAuth client from registry."""
        if client_name not in self.cfg.clients:
            raise OAuthException('Unconfigured client: %s' % client_name)

        if client_name not in ClientRegistry.clients:
            raise OAuthException('Unsupported services: %s' % client_name)

        params = dict(self.cfg.clients[client_name], **params)
        return ClientRegistry.clients[client_name](**params)