Fixed namespace traversal for model's image field

If our model's image is nested deeper than the first level:

{
  'image_thumb': {
    'url': 'http://...',
    //...
  }
}

We want to be able to tell it to ImageGridList like this:

var me = this,
    gallery = me.down('image-grid-list-panel'),
    list = gallery.down('image-grid-list');
list.setUrlNameSpace('image_thumb.url');

So this commit fixes the object traversal.