Fix problematic CalendarTag <-> WeblogPlugin interaction

[udittmer]

As discussed recently on the mailing list, CalendarTag fails to link to a day that does have blog entries if the very first entry got deleted. This patch works around that by allowing for up to 3 blog entries (the pages with the ..._1, ..._2 and ..._3 endings) to have been deleted, and will still detect the 4th one. This number can be configured using the CalendarTag.NUM_PAGES_TO_CHECK constant.