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