Added startTime property to keep track of the time when the sound actually started playing music

Author: pavle-goloskokovic

v3/src/sound/webaudio/WebAudioSound.js

@@ -47,6 +47,13 @@ var WebAudioSound = new Class({
          * @property {number} playTime
          */
         this.playTime = 0;
+        /**
+         * [description]
+         *
+         * @private
+         * @property {number} startTime
+         */
+        this.startTime = 0;
         /**
          * An array where we keep track of all rate updates during playback.
          *