title: What are definesPresentationContext and providesPresentationContextTransitionStyle
body: <p><code>ViewController</code> has <code>definesPresentationContext</code> and  <code>providesPresentationContextTransitionStyle</code>,
  but have no idea about how they work.
  I watched the WWDC2011, but I'm still confused about these two things.</p>
  
  <p>Could anyone explain this, preferably with a simple demo?</p>