Support for returning full Album Art URI's

This patch allows the caller to specify that the album art URI that is returned is fully qualified (like the get_current_track_info interface does).  This means it can be used immediately without any manual processing.  This was sort of mentioned in the following conversation:

https://groups.google.com/forum/?hl=en#!topic/python-soco/z4HkdJfu8yk

The ``hasattr`` check and this line could be combined to

````
if not getattr(item, 'album_art_uri', False):
    return
````

It saves two lines and one level of indentation.