Add FlxSpriteGroup as a new addon

The new addon discussed here: #360

Huge thanks to @crazysam for providing the mockup code this is based on.

I tested it and it seems to be working just fine. I thought about adding some more values other than `x`, `y` and `alpha`, but those are probably the ones that are the most commonly used and the most helpful. Everything else can as well be achieved via `transformChildren()`.