Cache the mutable proxy of items w/in SSArrayDataSource

This improves performance as the heavy -mutableArrayValueForKey: lookup
isn't needed on every access of SSArrayDataSource#items (which is
relatively frequent).
Merge pull request #34 from andrewsardone/performance

Improve SSArrayDataSource scrolling performance
fix an unintended recursion
Provide a base implementation of numberOfItems
Add encoding for Travis