title: Disable UITabBarController animation while setting rootViewContorller
body: <p>I have a login screen and when user submit correct credentials i just change the root view controller of <code>window</code> to my <code>tabbarController</code>
  It shows a very abrupt animation like the screen is being bigger from top to bottom. I don't need any sort of animation when i logged in. How could i stop <code>UITabBarController</code> from being animate?</p>
  
  <p>Here is the method in my <code>AppDelegate</code> which i used to login.</p>
  
  <pre><code>func userDidLoggedIn() -&gt; Void {
          tabBarController = storyboard.instantiateViewControllerWithIdentifier("TabBarController") as! VaboTabBarController
  
  
          self.window?.rootViewController = self.tabBarController
          self.window?.makeKeyAndVisible()
  
          self.registerDeviceForPushNotification(UIApplication.sharedApplication())
  
      }
  </code></pre>
  
  <p>In my <code>LoginViewController</code> when credentials are accurate i just use below code to switch to <code>TabBarController</code>.
  Here is the code.</p>
  
  <pre><code>dispatch_async(dispatch_get_main_queue(), {
                          UIView.animateWithDuration(0.9, animations: {
                              let userDefault = NSUserDefaults.standardUserDefaults()
                              let array = parseJSON["userData"] as! NSArray
                              appDelegate.userDidLoggedIn()
                          })
  </code></pre>