title: UIButton with transparent background image is not working as it should
body: <p>I created UIButton with transparent PNG. </p>
  
  <pre><code> instantHelpBtn = UIButton()
   instantHelpBtn.frame = CGRectMake(10, 10, 30, 30)
   instantHelpBtn.setBackgroundImage(UIImage(named: "questions1.png"), forState: .Normal)
   instantHelpBtn.addTarget(self, action: "instantHelpBtnPressed", forControlEvents: .TouchUpInside)
   self.view.addSubview(instantHelpBtn)
  </code></pre>
  
  <p>The problem is that function <code>instantHelpBtnPressed</code> is working only if I'm somehow pressing non-transparent parts of the background image. If I miss (and I miss often), nothing happens. How could I change this behavior?</p>