Give a UIAccessibilityTraitSelected to the selectedButton

Currently, the selected date is the calendar view doesn't have the UIAccessibilityTraitSelected trait. It affects VoiceOver accessibility and testing using accessibility traits.

This change ensure the selected date always has the UIAccessibilityTraitSelected trait when displayed.
Make -tableView:heightForRowAtIndexPath: faster and linear in time, refs #6

Mostly an implementation of DwightEverhart sample code (https://github.com/square/objc-TimesSquare/issues/6#issuecomment-13690845)
Merge branch 'uiaccessibilitytraitselected'
Merge branch 'optimize-height-calculations'
Update Xcode upgrade checks for Xcode 4.6
Merge branch 'xcode-46-upgrade-checks'