Ember.ajax

I did talk with @wycats, and I had an ok to implement this.

If this PR is usefull, I still have some doubts, like:

* Is the package `jquery-extensions` apropriated ?
* The ajax request isn't just a RSVP.Deferred, is actually a Ember.Object with Ember.Evented and Ember.DeferredMixin, so in some moment the destroy must be called?

Thanks in advance.