CRUD->addRef, custom model for sub-View

Now CRUD->addRef can use custom models for sub-View.
That's especially handy when you need deep references, for example, in case of n:m relationship.

Imagine models: `Client --< DocumentRelation >-- Document`.
I have CRUD of clients and I want to add expander for CRUD of Document (not DocumentRelation).
So now I can do simply like this:

```
$sub = $client->addRef('Document', array(
    'view_model' => function($model) {
        $d = $model->add('Model_Document');
        $d->addCondition($d->id_field, 'in', $model->ref('DocumentRelation')->fieldQuery('document_id'));
        return $d;
    },
));
```