Use factory superclass in Route#model hook

With factory injections, container objects are now extended when registered. 

This results in: `container.lookupFactory('model:post') !== App.Post`

Which in Ember Data, means:

```javascript
var Post = container.lookupFactory('model:post');
Post === App.Post; // false
Post.find(1) === App.Post.find(1); // false
```

Since currently most Ember apps use `App.Post.find(1)` or `store.find(App.Post, 1)`, this will cause Ember RC8 to break all apps that use Ember Data or access models in a similar way.

This PR is a quick workaround that prevents the RC8 release from having this breaking change.  An Ember Data PR will also follow with a similar fix for internal lookup of models.

Any better ideas for a quick fix are welcome (I'm not very happy with this one).

### Issues with this fix
- It is obviously a hack that needs to be re-visited at a later stage.
- Ember Data's update will cause it to be incompatible with Ember < RC8.
- Model injections would no longer be possible.