Add item_id to MusicLibraryItem taken from XML attribute
(create_sonos_playlist): Fix return for MLSonosPlaylist
(class MusicLibraryItem): Make item_id a required named arg
Remove item_id from subclasses
Update all tests to use item_id from xml id attr
lint
(browse): Fix MusicLibraryItem constructor
Add :item_id doc string to MusicLibraryItem constructor
Remove MLArtist __init__ method
Merge branch 'master' into itemid-from-xml
Attempt to fix browse_by_idstring to create an item_id

The MusicLibraryItem uri field isn't ever used by browse, so
search_uri is probably not needed.
Remove MLSameArtist call to superclass constructor.
Merge remote-tracking branch 'upstream/master' into itemid-from-xml
Fix create_sonos_playlist_from_queue
Merge pull request #233 from dajobe/itemid-from-xml

Get item ID from XML