Extracted Navigation Controller creation to a separate method.

For most customization needs, accessing the `navigationController` property of the assets picker will be sufficient. But you may need to specify a custom UINavigationController subclass if you want to control things like the status bar:

```objective-c
- (UIStatusBarStyle)preferredStatusBarStyle
{
    return UIStatusBarStyleLightContent;
}
```

Or maybe you already have a `BaseNavigationController` you would prefer to use instead of the stock one.

This Pull Request adds a `-createNavigationController` method that can be easily overridden to return your custom Navigation Controller subclass:

```objective-c
@interface BaseAssetsPickerController : CTAssetsPickerController 
@end

@implementation BaseAssetsPickerController
- (UINavigationController *)createNavigationController
{
    return [BaseNavigationController alloc];
}
@end
```