SSSectionedDataSource

Justice League, assemble! @segiddins @andrewsardone @basitali

I would love your feedback and comments on `SSSectionedDataSource`, a new addition to the `SSDataSources` family.

`SSSectionedDataSource` models a table or collection view with multiple sections. It is a subclass of `SSBaseDataSource` -- not `SSArrayDataSource` as I felt their APIs are too different.

Each section is modeled with an `SSSection` object, which stores its items along with a header/footer string.

I've updated the Example project to include a sectioned table example. You can drag/reorder items between sections, add and remove items and have sections handled for you.

Any architectural concerns? Missing APIs or features? Thanks all!