Add FlxTypedSpriteGroup
Merge branch 'dev' of https://github.com/HaxeFlixel/flixel into FlxTypedSpriteGroup

Conflicts:
	flixel/group/FlxSpriteGroup.hx
FlxTypedSpriteGroup: opimization via statically typed _sprites array
FlxTypedSpriteGroup: also use _sprites for reset()
Merge branch 'dev' of https://github.com/HaxeFlixel/flixel into FlxTypedSpriteGroup

Conflicts:
	flixel/group/FlxSpriteGroup.hx
FlxTypedSpriteGroup: optimize transformChildren() and multiTransformChildren()
FlxTypedSpriteGroup: optimization for add()
FlxTypedGroup: slightly nicer syntax for multiTransformChildren()
FlxTypedSpriteGroup: cleanup for reset()
Merge pull request #876 from Gama11/FlxTypedSpriteGroup

Add FlxTypedSpriteGroup