Add test for ES6 Promises

[This feature test is from Jake Archibald's ES6 Promises polyfill](https://github.com/jakearchibald/ES6-Promises/blob/master/lib/promise/polyfill.js).

Quick browser test:
Firefox 26 returns `false`
Chrome 31.0.1650.63 returns `false`
Chrome 34.0.1766.0 canary returns `true`