place background view behind the foreground view when inserting subviews

I a using QMBParallaxScrollViewController almost identically to your 'Map View Example'. I wanted to create a partially transparent overlay that would sit on top of my map (the `backgroundView`) and which would be a part of and would scroll with my `foregroundScrollView`. The `backgoundView` was in fact being inserted above the `foregroundScrollView`, so my image overlay was sitting behind the map instead of on top of it. This simple change fixes that and allows people to have content in their `foregroundScrollView` that sits on top of their `backgroundView`.