Destroyable

I noticed that chained animations have a destroy function while single animations does not.
As both are animations, it would be nicer to have the same API.
I propose to add destroy() on all animations.

Additionally, animations should be stopped when a widget is destroyed. Else, the animation may throw an error because the domNode doe not exist anymore.
I propose to do an anim.stop() in the destroy of the animation.

Thanks to both changes, we can use own() to handle the destroy/stop of the animation.