BUGFIX: In removeTween(), check that _activeTweens contains the tween to be removed.

BUG: Calling removeAllTweens() in an onComplete function will cause all
tweens to be removed from the _activeTweens list. When removeTween() is
then called in the GoKitLite Update() function, that tween is no longer
in the list and  _activeTweens.RemoveAt() will throw an exception.
Merge pull request #6 from bncastle/master

In removeTween(), check that _activeTweens contains tween to be removed.