code: function(object, object_id) {
      var res = {
        client_id:     this._credentials.client_id,
        client_secret: this._credentials.client_secret,
        object:        object,
        aspect:        'media',
        verify_token:  'hummingbird',
        callback_url:  this._callback
      };
      if (object_id) res.object_id = object_id;
      return res;
    }
docs: Generate request parameters for subscription