docs: def register_token(platform, user_id, token, on_error=None, on_success=None):
      """ Register a device token for a user.
  
      :param str platform The platform which to register token on. One of either
      Google Cloud Messaging (outbound.GCM) or Apple Push Notification Service
      (outbound.APNS).
  
      :param str | number user_id: the id you use to identify a user. this should
      be static for the lifetime of a user.
  
      :param str token: the token to register.
  
      :param func on_error: An optional function to call in the event of an error.
      on_error callback should take 2 parameters: `code` and `error`. `code` will be
      one of outbound.ERROR_XXXXXX. `error` will be the corresponding message.
  
      :param func on_success: An optional function to call if/when the API call succeeds.
      on_success callback takes no parameters.
      """
      __device_token(platform, True, user_id, token=token, on_error=on_error, on_success=on_success)