Make sure row >= 0 to prevent crash when using toggle button to toggle on/off displaying year

When year is off then toggled on, row ends up hugely negative (because minimumYear is subtracted from year in rowForYear: but year == 1 when year is toggled off) and middleBlockIndex is 0, resulting in crash
PMEDatePicker: Updated to determine correct number of days in month given month and year
PMEDatePicker: Hack to permit Feb 29 to be acceptable when year is not enabled