Don't require artistId field [#153]
Don't require artistId field [#153]

TheOpenDevProject/gmusicapi,thebigmunch/gmusicapi,dvirtz/gmusicapi,peetahzee/Unofficial-Google-Music-API,simon-weber/gmusicapi,dvirtz/gmusicapi,tanhaiwang/gmusicapi,simon-weber/Unofficial-Google-Music-API