Support for returning full Album Art URI's

Pull request to replace #187 which I made a mess of with git again!

This should include all the proposed changes that were listed in #187 

Description from #187 

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

Please can this be flagged for 0.9