Change of API for Album Art

[naggie]

Later on, for scalability reasons, it may be necessary to host album art using the web server instead of the somewhat non-standard blob-serialisation approach currently used.

For the interim, the albumArtId JSON attribute will change to AlbumArtUrl prior to this change.

That way, changing how the album art is served won't affect any clients out there providing they use AlbumArtUrl before the first release.