WIP: Switch from jQuery promises to RSVP

### Description
Total Work In Progress here, any comments  welcome. I've gotten the almost all the tests passing under Ember 1.0. I couldn't immediately figure out an elegant way to get at the Server Response after casting the jQuery ajax promise to RSVP, so the two tests relying on that are still failing, I should have to time to solve that somehow tomorrow.

My updates to the README.md should properly reflect what's happened here, most things work the same but you start getting a Promises/A+ promise and you don't get the Resource back when your fetch resolves. These changes are probably too big to get merged before 3.0 but let me know what you think.

I wasn't really sure what the `fetched` is for, I have tried to maintain the behaviour anyway, returning something which will resolve *only once*, despite multiple fetches.

Also, this would solve https://github.com/zendesk/ember-resource/issues/77 "for free", yay Promises/A+ =)

cc @shajith @jamesarosen