Fix generic constraints on UICollectionView's supplementary views

• Added documentation
• Fixed generic constraints on the API working with UICollectionView's SupplementaryView
• Updated Example project to add an UICollectionView with cells from XIB & Code + Header views

First stable release

• Split protocol in two: Reusable and NibReusable
• Now contains a Demo project

Initial version

Initial version. Only one Reusable protocol
No demo project, but has a podspec and a Package.swift