v0.5.0

Add support for scrolling in header #48

• Require PagingItem to conform to Hashable and Comparable: fbd7aff
• Write custom collection view layout instead of using UICollectionViewFlowLayout c6f78b4

Note: With this change, Parchment requires PagingItem to
conform to both Hashable and Comparable. This means any
existing PagingItem implementation have to be updated.