v0.4: Supplementary view headers and footers

https://cocoapods.org/pods/CellKit

• Rename registered headers to supplementary views
• Check uses NIB in registration
• Add supplementary view registration and presentation for flow layout
• Fix Swift version for CocoaPods 1.6
• Conform to flow delegate in abstract data source
• Add collection reusable view to prevent inconsistencies
• Add init of empty data source
• Fix cell model data source init super calls