listIssueComments(issue, cb) {
      return this._request('GET', `/repos/${this.__repository}/issues/${issue}/comments`, null, cb);
   }
List comments on an issue
@see https://developer.github.com/v3/issues/comments/#list-comments-on-an-issue
@param {number} issue - the id of the issue to get comments from
@param {Requestable.callback} [cb] - will receive the comments
@return {Promise} - the promise for the http request