Fixes #215

[Skehmatics]

Fixes #215 by iterating through _active rather than _animations. This also could make canceling per-cookie/handler possible:

cancelAnimation: function(cookie) {
  for (var i in this._active[cookie]){
    this._active[cookie][i].animation.cancel();
  }
  delete this._active[cookie];
}

Tested in Chrome 55.

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[Skehmatics]

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