创建 Empty Application时， storyboard 不显示的问题

原因： 1. 创建 Empty Application 时没有 Using Storyboard 选项，需要手动创建 MainStoryboard。
          2. Changed Main Storyboard in Target's Summary view
          3. For an Empty Application, the default implementation of  didFinishLaunchingWithOptions is :

  - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]] autorelease];
    // Override point for customization after application launch.
    self.window.backgroundColor = [UIColor whiteColor];
    [self.window makeKeyAndVisible];
    return YES;
}

    原因：此处重新创建的 Window 把我们在 storyboard 里的 UI 给覆盖了。

    4.  update the default implementation of didFinishLaunchingWithOptions to the following:
    - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    return YES;
}

    BTW： if we create an Sing View Application, we can see that "Using storyboard" checkbox.