message: emit an event when a context is closed
  to clean up Master/Transport and anything else static on the context
diff: @@ -337,6 +337,7 @@ define(["Tone/core/Tone", "Tone/core/Emitter", "Tone/core/Timeline"], function (
  * @returns {Tone.Context} this
  */
  Tone.Context.prototype.dispose = function(){
  + Tone.Context.emit("close", this);
  Tone.Emitter.prototype.dispose.call(this);
  this._worker = null;
  this._timeouts.dispose();
  