add get_key and get_all_keys interface
add get_key and get_all_keys interface

thebigmunch/gmusicapi,simon-weber/gmusicapi,dvirtz/gmusicapi,nvbn/Unofficial-Google-Music-API,peetahzee/Unofficial-Google-Music-API,simon-weber/Unofficial-Google-Music-API,jimyx17/gmusic,TheOpenDevProject/gmusicapi,dvirtz/gmusicapi,tanhaiwang/gmusicapi