listPullRequests(options, cb) {
      options = options || {};
      return this._request('GET', `/repos/${this.__fullname}/pulls`, options, cb);
   }
List the open pull requests on the repository
@see https://developer.github.com/v3/pulls/#list-pull-requests
@param {Object} options - options to filter the search
@param {Requestable.callback} [cb] - will receive the list of PRs
@return {Promise} - the promise for the http request