Register a new Subscription on this collection's parent object.

        Args:
          callback_url (str): URI of an active endpoint which can receive
          notifications.

        Returns:
           A round.Subscription object if successful.

def create(self, callback_url):
        """Register a new Subscription on this collection's parent object.

        Args:
          callback_url (str): URI of an active endpoint which can receive
          notifications.

        Returns:
           A round.Subscription object if successful.
        """
        resource = self.resource.create({'subscribed_to': 'address',
                                         'callback_url': callback_url})
        subscription = self.wrap(resource)
        self.add(subscription)
        return subscription