title: Make UICollectionView always stop scrolling with item centered
body: <p>I want, basically, something that works like the task switching UI, where no matter how you scroll, an item always ends up perfectly centered.  This is different than paging, I think, since more than one item will be visible (the centered item, and the item to the left and right, if they exist).</p>
  
  <p>How could I achieve this (iOS 7+).</p>