Promises Step 1: Use promises instead of callbacks
This step was a small step, merely to swap out
callbacks for promises at the adapter level.
Subsequent steps will result in more aggressive
refactoring, and begin to fully use promises.

@stefanpenner & @leepfrog aka 'the leeping Penner'

Bonus:
```Model.find#then``` now rejects/resolves correctly.
_deferred and promise should not be accessed outside the mixin
Merge pull request #3 from teddyzeenny/load-promise

_deferred and promise should not be accessed outside the mixin
Merge branch 'master' of https://github.com/emberjs/data

Conflicts:
	packages/ember-data/lib/adapters/rest_adapter.js
Merge pull request #934 from stefanpenner/promisify

Promises Step 1: Use promises instead of callbacks