Added support for Sectioned Lists

Call this SectionListItemGenerator since it is limited to just SectionList

And we should document this better by saying something about grouping the list items into sections and then adding an item for the header for each section followed by item for each element in that section.

As far as design is concerned, I had assumed that something would take in a List of Items and return List of ItemSections. Each Section has a List of Items and a single Header Data. 

The task of flattening this out should be with the Adapter and internal to it.