Bug 839838 - Implement DOMRequest.then; r=sicking,bzbarsky

This is implemented by creating a Promise object internally and
forwarding the .then() call to it. Any further callbacks passed to
future .then() calls will be added as callbacks on the same internal
promise object. We also take care of resolving or rejecting the promise
if the success/error event of the DOMRequest object has been fired
before .then() is called.