added shadow and option for Views

Like @RomainBoulay I added support for shadows in views. This commit includes both a shadow option and ensures the shadow is presented. In my case the changes @RomainBoulay has made in his push request https://github.com/apping/APParallaxHeader/pull/22  didn't show any shadow on top of my custom view. Only bringing the shadowView to the front (like I did here) works for me.