Based on:
- https://github.com/brightec/CustomCollectionViewLayout
- https://github.com/bryceredd/RFQuiltLayout
- https://developer.apple.com/library/ios/documentation/WindowsViews/Conceptual/CollectionViewPGforIOS/CreatingCustomLayouts/CreatingCustomLayouts.html#//apple_ref/doc/uid/TP40012334-CH5-SW1
- https://developer.apple.com/library/ios/documentation/UIKit/Reference/UICollectionViewLayout_class/index.html#//apple_ref/occ/cl/UICollectionViewLayout