Subscribes at the server.

def create(self):
        """
        Subscribes at the server.
        """
        self.logger.debug('Create subscription on server...')

        if not self.connection.connected:
            self.state = 'connection_pending'
            return

        data = {
            'command': 'subscribe',
            'identifier': self._identifier_string()
        }

        self.connection.send(data)
        self.state = 'pending'