Cells are not added in the same order as layoutAttributesArray

The standard UICollectionViewController implementation adds cells beginning in the top-left and ending in the bottom-right. In PSTCollectionView, the cells are added in the order defined by `itemKeysToAddDict enumerateKeysAndObjectsUsingBlock` in [PSTCollectionView.m](https://github.com/steipete/PSTCollectionView/blob/master/PSTCollectionView/PSTCollectionView.m#L1322), hence no particular order.

While this isn't so much of a problem for fast loading resources, the order becomes more important when sequential loading of images is employed.

This pull request iterates through `layoutAttributes` instead of enumerating `itemKeysToAddDict`, therefore preserving the order.