Implement artist-scope (individual tracks or grouped by album) shuffle

src/pkjs/index.js

@@ -393,6 +393,7 @@ function callAPI(method, endpoint, callback) {
     xhr.onload = function () {
         switch (xhr.status) {
             case 200:
+            case 204:
                 if (callback) {
                     callback(this.responseText);
                 }