performance fix for slow laggy scrolling.

NSDateFormatter has a big overhead for instantiating.   If you run in the profiler, you will see that it's using a huge amount of time.

If you make sectionFormater a property of CALAgendaViewController, initialize it in viewDidLoad and reference it from -updateEventGroupByDays and -eventsAtIndexPath: you will see smooth scrolling, even on 4S iPhone.