Allow FlxSpriteGroup to have FxlSpriteGroups as elements.

 https://github.com/HaxeFlixel/flixel/issues/391
Refactoring - moved IFlxBasic to FlxBasic.hx and IFlxSprite to FlxSprite.hx
Merge pull request #580 from SergioB/dev

Allow FlxSpriteGroup to have FlxSpriteGroups as elements, closes #391