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

It's not - I made a mess with github again! :-(