getIssue(issue, cb) {
      return this._request('GET', `/repos/${this.__repository}/issues/${issue}`, null, cb);
   }
Get a particular issue
@see https://developer.github.com/v3/issues/#get-a-single-issue
@param {number} issue - the issue number to fetch
@param {Requestable.callback} [cb] - will receive the issue
@return {Promise} - the promise for the http request