createPullRequest(options, cb) {
      return this._request('POST', `/repos/${this.__fullname}/pulls`, options, cb);
   }
Create a new pull request
@see https://developer.github.com/v3/pulls/#create-a-pull-request
@param {Object} options - the pull request description
@param {Requestable.callback} cb - will receive the new pull request
@return {Promise} - the promise for the http request