Image category / properties

In current [Images api](http://auraspec.readthedocs.org/en/latest/api.html#images), images are only identified by an index and associated library item. I believe this will be too restricting on a real use case:
- clients not always display the same image on a table view and details view. I think image should be categorized and browse by category if requested: cover, fanart, background, banner, ...
- sometimes it's better to display an image matching better the device screen size/resolution, we should be able to know/query an associated image by width/height.