Added previousTime property to keep track of audio tag's playback position recorded on previous update method call

Author: pavle-goloskokovic

src/sound/html5/HTML5AudioSound.js

@@ -27,6 +27,15 @@ var HTML5AudioSound = new Class({
          * @default null
          */
         this.audio = null;
+        /**
+         * Audio tag's playback position recorded on previous
+         * update method call. Set to 0 if sound is not playing.
+         *
+         * @private
+         * @property {number} previousTime
+         * @default 0
+         */
+        this.previousTime = 0;
         /**
          * Timestamp as generated by the Request Animation Frame or SetTimeout
          * representing the time at which the delayed sound playback should start.