http://v.youku.com/v_show/id_XMTcxMzEyOTAzNg==.html?beta&
CHAPTER 17: Animated and Interactive Collection Views
add Gesture call invalidateLayout()
func prepareForCollectionViewUpdates(updateItems: [UICollectionViewUpdateItem])
func initialLayoutAttributesForAppearingItemAtIndexPath(itemIndexPath: NSIndexPath) -> UICollectionViewLayoutAttributes?
func finalLayoutAttributesForDisappearingItemAtIndexPath(itemIndexPath: NSIndexPath) -> UICollectionViewLayoutAttributes?
