New SPGroup class and little adds in animation classes

[Abawell]

Added new SPGroup class.

The SPGroup takes objects that implement SPAnimatable (e.g. SPTweens) and executes them, just like SPJuggler does but objects can be animated sequentially or in parallel. A SPGroup can be added to a SPJuggler or to an other SPGroup. SPGroup is very simple but makes complexe animations really easier.

If you don't like the name, just change it.

Added 'isEmpty' property to SPJuggler. I needed it in my application.

Added 'show', 'hide' and 'removeFromParent' methods to SPTween. Could be done by onStart and onComplete blocks but I think it's just butter like that.

[PrimaryFeather]

Hi Jérôme, thanks a lot for the pull request, and sorry for the late reply! I've got another list of changes from Robert Carone on my list; when I'm merging those, I'll look into your changes, as well. Thanks for your patience, and thank you for taking the time to write the pull request! :smile: Daniel