title: cannot set background = UIColor.clearColor() for UIBarButtonItem
body: <p>I can set <code>clearColor</code> for <code>UIButton</code> </p>
  
  <pre><code>var btn = UIButton()
  btn.backgroundColor = UIColor.clearColor() 
  </code></pre>
  
  <p>But I can't set this for <code>UIBarButtonItem</code>. </p>