Start of the model part of the BHPhotoAlbum UICollectionView tutorial
Revert "Start of the model part of the BHPhotoAlbum UICollectionView tutorial"

This reverts commit 5918d83f69d185ccaafca6cd7491bd87aa11ea31.
Revert "Revert "Start of the model part of the BHPhotoAlbum UICollectionView tutorial""

This reverts commit 47fcb3d849e0d2de37155f0c4e03fcdbeeb453f0.
Tutorial Asynchronous loading

Sstill filled with tutorial stuff, not the real guts. tutorial includes
asynchronous loading of cell content. Super!