Got rid of single var tweens, MultiVarTween -> VarTween

also
  - added some safety checks to VarTween.tween()
  - removed an unnecessary null check from the update() loop
Fix some issues with 64c7a418b8f35f8badca143b42d407b8c38c0c31

thanks @gamedevsam
Merge branch 'dev' of https://github.com/HaxeFlixel/flixel into dev
Merge branch 'dev' of https://github.com/SeiferTim/HaxeFlixel into dev
Tweaking `FlxPreloader` and removing now-unessecary Debug logic.