Define ASCollectionView interface, ASCollectionViewDataSource and ASCollectionViewDelegate protocols.
Implement ASCollectionView. Horizontal scrolling is not supported yet.
Fix ASCollectionView::rangeControllerVisibleNodeIndexPaths: to return a sorted array.
Provide ASCollectionView::calculatedSizeForNodeAtIndexPath:, to be used in custom UICollectionViewLayouts.
Make constrained size for nodes adapt to ASCollectionView's scrolling direction.
Update ASRangeController to support ASCollectionView and no longer assume vertical scrolling when calculates working range.
Add CollectionViewSample.

Update sample for more items.
Merge pull request #101 from nguyenhuy/collection_view

Add ASCollectionView