Improve SSArrayDataSource scrolling performance

In an app I’m working on, I noticed a not insignificant amount of time was being [spent in `-[SSArrayDataSource items]][ss] due to the constant `-mutableArrayValueForKey:` message send.

This change simply internally caches the mutable array within `SSArrayDataSource`.

- [primitive trace document][idoc] with a before and after run using the a [100 rows][1r] Example app

[ss]: https://cloud.githubusercontent.com/assets/14554/2980091/23433b12-dbe0-11e3-8b0f-07cde51986e0.png
[idoc]: http://cl.ly/3z253y0t0Z0D
[1r]: http://cl.ly/code/2o381P2c3c46

refs splinesoft/SSDataSources#29