Set shadow properties after view has been instantiated.

I'm building a universal app where I show the shadow on iPad and hide the shadow on iPhone. I achieve this by subclassing the first view controller for each device where the necessary customisations are performed.

``` objc
@implementation DirectionViewController_iPhone

// snip

- (void)viewDidLoad {
    [super viewDidLoad];
    
    // snip    

    self.revealViewController.frontViewShadowOpacity = 0.0;
    [self.revealViewController panGestureRecognizer];
    [self.revealViewController tapGestureRecognizer];
}
```

This means that by the time the subclass is performing customisation in `viewDidLoad` the `SWRevealView` has already been instantiated and ignores any changes to the shadow properties. This patch lets you customise the shadow after the view has been instantiated.

I also noticed that @iferrera had the same issue in #223. I'd love to see this merged into master and welcome any feedback you have.