Make `RSVP.all` work with more than just promises.
Add `RSVP.denodeify`.

Converts functions that use the Node.js-style callbacks as their last parameter into promise-returning functions. Such functions also accept promises, in order to allow cool composition (see tests for example).
Merge branch 'promisify' of https://github.com/domenic/rsvp.js into domenic-promisify