createIssueComment(issue, comment, cb) {
      return this._request('POST', `/repos/${this.__repository}/issues/${issue}/comments`, {body: comment}, cb);
   }
Comment on an issue
@see https://developer.github.com/v3/issues/comments/#create-a-comment
@param {number} issue - the id of the issue to comment on
@param {string} comment - the comment to add
@param {Requestable.callback} [cb] - will receive the created comment
@return {Promise} - the promise for the http request