Add collectionView:didRemoveCell:forItemAtIndexPath: message to JNWCollectionViewDelegate

A message to notify the delegate when cells go off-screen. I have missed this for cells that need to have work started asynchronously and cancelled when they are scrolled through before the task finishes.
I think a better name for this delegate method would be something like how `UICollectionView` does it. Namely, `-collectionView:didEndDisplayingCell:forItemAtIndexPath:`.