…onViewItem. Fixes #71
* Now extremities can overlay items in their section <http://soff.me/AKVa> * Also added `- (void)collectionView:(SSCollectionView *)aCollectionView willDisplayItem:(SSCollectionViewItem *)item atIndexPath:(NSIndexPath *)indexPath` to SSCollectionViewDelegate
SSCatalog was using a vendored version of JMImageCache which had several flaws. Even though AFNetworking is a bit heavier dependency-wise, I am using that in my example because it is strongly preferred over JMImageCache.