title: UINavigationBar appearance in Swift programmatically don't work
body: <p>I am using this code for set backgroundImage NavigationBar...The navBar doesn't appear just!</p>
  
  <p>I am using Swift with Xcode 6.1</p>
  
  <p>What is wrong?</p>
  
  <pre><code>    let vista: UIViewController = Vista_Principale()
      let categorie: UITableViewController = Vista_Categorie()
      let preferiti: UITableViewController = Vista_Preferiti()
      let info: UIViewController = Vista_Info()
      var nav = UINavigationController(rootViewController: vista)
      var nav2 = UINavigationController(rootViewController: categorie)
      var nav3 = UINavigationController(rootViewController: preferiti)
  
  
      UINavigationBar.appearance().setBackgroundImage(immagine_NavBar, forBarMetrics:.Default)
  
  
      let tab_Bar = UITabBarController()
      let viste = [vista,categorie,preferiti,info]
      tab_Bar.viewControllers = viste
      tab_Bar.tabBar.backgroundImage = UIImage(named: "Tab_Bar")
      tab_Bar.tabBar.tintColor = UIColor.whiteColor()
  
  
  
  
      self.window!.rootViewController = tab_Bar
  
      self.window!.makeKeyAndVisible()
  
  
      return true
  
  }
  </code></pre>