Adds Promise#race

There's one failing test `resolves an empty array passed to RSVP.race()`. Domenic [commented](https://github.com/tildeio/rsvp.js/pull/102/files#r6185903) on [any PR](https://github.com/tildeio/rsvp.js/pull/102) saying that "if there are no promises, then there are no promises that ever satisfy your desire, so you should return a forever-pending promise." That's not how `all` works for example and I feel like we should be consistent.

Please correct me if I don't understand something here.

/cc @domenic