feature(repository): add listPullRequestFiles (#353)

Author: AurelioDeRosa

lib/Repository.js

@@ -108,6 +108,17 @@ class Repository extends Requestable {
       return this._request('GET', `/repos/${this.__fullname}/pulls/${number}`, null, cb);
    }
 
+   /**
+    * List the files of a specific pull request
+    * @see https://developer.github.com/v3/pulls/#list-pull-requests-files
+    * @param {number|string} number - the PR you wish to fetch
+    * @param {Requestable.callback} [cb] - will receive the list of files from the API
+    * @return {Promise} - the promise for the http request
+    */
+   listPullRequestFiles(number, cb) {
+      return this._request('GET', `/repos/${this.__fullname}/pulls/${number}/files`, null, cb);
+   }
+
    /**
     * Compare two branches/commits/repositories
     * @see https://developer.github.com/v3/repos/commits/#compare-two-commits