code: function (callback) {
  
      var promise = this.request({
        method: 'delete',
        url: 'users/authenticate'
      }).then(this.authorize.bind(this));
  
      if (callback) {
        promise.then(callback.bind(null, null)).fail(callback);
      }
  
      return promise;
    }
docs: De-authenticate the Vine API client
  @param username {String}
  @param password {String}
  @param callback {Function} [optional]
  @return {Object} request API promise