Revert "place background view behind the foreground view when inserting ...

Reverts quemb/QMBParallaxScrollViewController#16

After merging it, I remembered why I choose to place the _backgroundView on top of the _foregroundScrollView. Even though it sounds strange, but I needed user interaction on the _backgroundView. And because you need user interaction enabled on the _foregroundScrollView, the background view needs to be on top.

But I can image to implement a config option for that..