$pkField should support multimensional fields

the idea is to have a multimensional object as result of the REST request.

``` javascript
[{
 modelData : { id : 1234 },
 content : { ... }
},
[{
 modelData : { id : 1235 },
 content : { ... }
}
]
```

This way the content of the item is in one place and the id on another in our case id is our $pkField.

ideas:

``` javascript
{
  $pkField : "modelData.id"
}
```
