Added Shadow with option + Improved Interface Orientation Behavior with new Options

[Jeehut]

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 #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.

[Jeehut]

Starting from the latest stable version (0.1.6) I also now fixed several problems with change of interface orientation. Now there are two new options:

1. Choose a different height for landscape and portrait modes.
2. Choose a ratio with which the height should be automatically calculated in relation to the width.

In order to the interface orientations to take effect there's a new method named rotateParallaxViewToInterfaceOrientation that must be called in the ViewController. The following code worked for me (using Swift and XCode 6.1.1):

override func willAnimateRotationToInterfaceOrientation(toInterfaceOrientation: UIInterfaceOrientation, duration: NSTimeInterval) {

        UIView.animateWithDuration(duration, animations: nil, completion: { success in
            self.collectionView?.rotateParallaxViewToInterfaceOrientation(toInterfaceOrientation)
        })
    }