Adds Promise.defer() for router.js compatibility

I'm [looking for a simple Promise implementation for using with router.js](https://github.com/tildeio/router.js/issues/82), first I've looked at `then/promise` but has features (nodejs related) I don't need for.

Actually, router.js is using `RSVP.defer()` for some internal handling, after patching this everything seems to be working perfectly.

What do you think?