Use NSWeekOfMonthCalendarUnit not NSWeekOfYearCalendarUnit

The selection wasn't showing in January 2012 in the en_GB locale for the 3rd and subsequent weeks of the month.  This was due to `-indexPathForRowAtDate:` using NSWeekOfYearCalendarUnit which behaves differently in en_GB to en_US. This fixes the issue by using NSWeekOfMonthCalendarUnit instead.