Implement base item methods in SSArrayKeyPathDataSource

* -[SSArrayKeyPathDataSource itemAtIndexPath:]
* -[SSArrayKeyPathDataSource numberOfItems]
* -[SSArrayKeyPathDataSource numberOfSections]
* -[SSArrayKeyPathDataSource  numberOfItemsInSection:]
* Other tweaks
Extract array item access into a protocol

This allows for a shared interface between array-backed SSDataSource
objects when it comes to individual item access.
Implement SSArrayItemAccess on SSArrayKeyPathDataSource

Note, the NSManagedObjectID look-up isn't tested because there isn't a
handy Core Data test stack. Hopefully I'll get around to this later.
Add tests for basic SSArrayKeyPathDataSource manipulation
Implement KVO responses for manipulating cells
Add docs for SSArrayKeyPathDataSource