Adds SoCo.get_sonos_playlists and MLSonosPlaylist

based on comments in https://github.com/SoCo/SoCo/issues/113


to test it

```
lists = sonos.get_sonos_playlists()
print("search_type: %s number_retured: %s update_id: %s total_matches: %d" % (
 lists.get('search_type'),
 lists.get('number_retured'),
 lists.get('update_id'),
 lists.get('total_matches')))

for idx, playlist in enumerate(lists.get('item_list'), 1):
    print("idx: %d, playlist: %s" % (idx, playlist))
```


I'm not sure yet why the unit-test is failing.