listPullRequestFiles(number, cb) {
      return this._request('GET', `/repos/${this.__fullname}/pulls/${number}/files`, null, cb);
   }
List the files of a specific pull request
@see https://developer.github.com/v3/pulls/#list-pull-requests-files
@param {number|string} number - the PR you wish to fetch
@param {Requestable.callback} [cb] - will receive the list of files from the API
@return {Promise} - the promise for the http request