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