getPullRequest(number, cb) {
      return this._request('GET', `/repos/${this.__fullname}/pulls/${number}`, null, cb);
   }
Get information about a specific pull request
@see https://developer.github.com/v3/pulls/#get-a-single-pull-request
@param {number} number - the PR you wish to fetch
@param {Requestable.callback} [cb] - will receive the PR from the API
@return {Promise} - the promise for the http request