Fixed NSRangeException in TKCalendarMonthView

When traversing backwards by month, the element at index count +1 was accessed in the marks array, yielding an NSRangeException. I added a check to ensure that the index variable is not beyond the bounds of the marks array, and I also removed the redundant calls to [marks count] within the for-loops of -drawRect: of TKCalendarMonthView.m.