Hooks for record/collection extraction in RESTAdapter.

Basically I wanted to override the treatment of `rootKey`/`collectionKey`, something like:

```javascript
var OptionalRootKeyMixin = Ember.Mixin.create({
  extractData: function(klass, data, rootKeyProperty) {
    var rootKey = Ember.get(klass, rootKeyProperty);
    if (rootKey && data[rootKey]) {
      return data[rootKey];
    } else {
      return data;
    }
  }
});
```

But couldn't do this with the current hooks available. I think it might make sense to go a bit further and mimic the `RESTSerializer`[0] API, but wondered what people thought about this approach. Thanks!

[0]http://emberjs.com/api/data/classes/DS.RESTSerializer.html