Added song data class. Added trackers.

models/song.js

@@ -0,0 +1,16 @@
+class Song {
+  constructor(id, name, album, artists, year, duration, progress, session, state, source) {
+    this.id = id;
+    this.name = name;
+    this.album = album;
+    this.artists = artists;
+    this.year = year;
+    this.duration = duration;
+    this.progress = progress;
+    this.session = session;
+    this.state = state;
+    this.source = source;
+  }
+}
+
+module.exports = Song;