Add clampDate:toComponents:, and use it to clamp firstDate, lastDate
and selectedDate. Remove clamp from TSQCalendarRowCell, should have
been clamped by the time it gets here.
Remove contentOffsetForIndexPath:, since the two locations using
don't have enough in common, and using its offset was causing
scrolling to the next month if programmatically setting the selected
date to a date in the third row or later.