message: callback is invoked with the timeline as the context
diff: @@ -277,7 +277,7 @@ define(["Tone/core/Tone", "Tone/type/Type"], function (Tone) {
  lowerBound = this.defaultArg(lowerBound, 0);
  upperBound = this.defaultArg(upperBound, this._timeline.length - 1);
  for (var i = lowerBound; i <= upperBound; i++){
  - callback(this._timeline[i]);
  + callback.call(this, this._timeline[i]);
  }
  this._iterating = false;
  if (this._toRemove.length > 0){
  @@ -359,7 +359,7 @@ define(["Tone/core/Tone", "Tone/type/Type"], function (Tone) {
  if (upperBound !== -1){
  this._iterate(function(event){
  if (event.time === time){
  - callback(event);
  + callback.call(this, event);
  }
  }, 0, upperBound);
  }
  @@ -374,6 +374,7 @@ define(["Tone/core/Tone", "Tone/type/Type"], function (Tone) {
  Tone.prototype.dispose.call(this);
  this._timeline = null;
  this._toRemove = null;
  + return this;
  };
  return Tone.Timeline;
  