Tidied up the Signal class and added a call counter to SignalBinding.
Added onRemovedFromWorld signal. Not currently used anywhere, but will be.
Tidied up the jsdocs.
Buttons are now waiting to be told when they're removed from the World.
You can now call Tween.to again on a Tween that has already completed. This will re-use the same tween, on the original object, without having to recreate the Tween again. This allows a single tween instance to be re-used multiple times, providing they are linked to the same object (thanks InsaneHero)