mpris: Add artUrl support
mpris: Add discNumber support
Fix: Only add albumart if album name is defined
Merge branch 'soundcloud_backend' into develop
Merge branch 'develop' of git://github.com/mopidy/mopidy.git into develop
core: Warn if track isn't playable
docs: Add info about mailing list
models: Comment Album.images not preserving order
Use thread traceback logger from Pykka 1.1
Turn thread traceback logger on by default

It no longer requires an additional thread to work and it is very useful to
have this enabled when you hit an issue that is hard to reproduce.
Merge branch 'develop' of git://github.com/mopidy/mopidy.git into develop
Fix: SoundCloud api unavailable, leaves backend in broken state
Fix: Document issue with playlists for mobile clients
Fix: User is resolved before api exists
Merge branch 'soundcloud_backend' into develop