Made status bar tap scroll to current month and fixed header weekday abbreviation

Normally, the calendar scrolls to the first month shown in the calendar if the user taps the status bar. I thought that it might be more natural for it to scroll to the current month instead, so I overrode the ```(BOOL)scrollViewShouldScrollToTop:(UIScrollView *)scrollView``` delegate method to do just that.

I also noticed that the weekday abbreviations were inconsistent across the (iOS 6) simulator (which showed a three-letter abbreviation for each weekday, like the screenshots) and my (iOS 7) iPhone (which showed a two-letter abbreviation), so I modified the date format to show the three letter abbreviations consistently.