createMilestone(milestoneData, cb) {
      return this._request('POST', `/repos/${this.__repository}/milestones`, milestoneData, cb);
   }
Create a new milestone
@see https://developer.github.com/v3/issues/milestones/#create-a-milestone
@param {Object} milestoneData - the milestone definition
@param {Requestable.callback} [cb] - will receive the milestone
@return {Promise} - the promise for the http request