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 and performing any customisations as necessary.

``` objc
@implementation DirectionViewController_iPhone

// snip

- (void)viewDidLoad {
    [super viewDidLoad];
    
    // snip    

    self.revealViewController.frontViewShadowOpacity = 0.0;
    [self.revealViewController panGestureRecognizer];
    [self.revealViewController tapGestureRecognizer];
}
```

Because `SWRevealController` is a container controller, the `viewDidLoad` in my subclass is invoked after `SWRevealView` has been instantiated. Any changes to the shadow properties are silently ignored. This pull request 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.