Playlists endpoints addItemsToPlaylist() and removeItemsFromPlaylist() should return snapshot ID.

[danielvy]

addItemsToPlaylist() and removeItemsFromPlaylist() in PlaylistsEndpoints should return the playlist snapshot ID as returned by the WEB APIs: POST playlists/{playlist_id}/tracks and DELETE playlists/{playlist_id}/tracks

[danielvy]

Created #123 PR to fix this.