Fix first day in calendar error / number of rows in calendar.

When first day of month is sunday and option -use Monday as first day is set in calendar widget the month start form Monday. Problem when some month need more that 5 rows to show all days.
Merge branch 'patch-1' of https://github.com/tuure/silverstripe-event-calendar
See https://github.com/unclecheese/silverstripe-event-calendar/pull/24
a more general solution to issue unclecheese/silverstripe-event-calendar#24
Fix: Fixed issue unclecheese/silverstripe-event-calendar#26 (faulty
behaviour: show current month when selecting January through
MonthJumper)