Make Model implements ArrayableInterface

Otherwise `Cursor->toJson()` ends up calling json_encode directly on the models and the objects are serialized nested in an "attributes" key:

```class Annotation extends Model
{
    public $author;
    public $comment;
}```


 A cursor of these `Annotation`s will get serialized as a list of
```{
"attributes": {
    "author": "braboule",
    "comment": "braboule"
}```