Added item_id property to Sonos' playlist class

The original commit was made by David "DPH".
Made possible to create a Sonos' playlist

Allowed the creationg of new Sonos' playlists.
Made possible to add tracks to a Sonos' playlist
Fixed pylint errors and warnings
Updated inline documentation
Code refactoring

Renamed `add_track_to_sonos_playlist` to `add_item_to_sonos_playlist`.
Fixed broken test
Merge pull request #198 from flavio/sonos_playlist_write_operations

Sonos playlist write operations
Add Queue data structure and implements its use in core. Refactor container objects for more code reuse.
Doc string update after container object change
Merge pull request #214 from KennethNielsen/queue_object

Queue object