Add ES6 Promises

Promises are landing in the near future to browsers, so I think that now is a good time to add the feature to caniuse.

[Chrome/Blink is enabling Promises by default Promises in v32](http://www.chromestatus.com/features/5681726336532480). I could not find any info about support for other browsers, but at least Firefox and Safari are working on enabling support for Promises.

ES6 Promises API can be polyfilled to get cross browser support.

**More info:**

* [The ES6 promises spec](https://github.com/domenic/promises-unwrapping)
* [Promises/A+ spec](http://promises-aplus.github.io/promises-spec/)
* [JavaScript Promises: There and back again - HTML5 Rocks](http://www.html5rocks.com/en/tutorials/es6/promises/)
* [A polyfill for ES6-style Promises](https://github.com/jakearchibald/ES6-Promises)
* [RSVP.js changelog](https://github.com/tildeio/rsvp.js/blob/master/CHANGELOG.md)

ping @Fyrd