CalendarTruncator: Now also truncate week

Have been reluctant to adding this, but the scenarios where it's useful
are getting quite many.

The week used here is ISO 8601 weeks. Starting on Mondays, with week 01
being the week with the year's first Thursday in it.
Merge pull request #39 from iZettle/truncate-weeks

CalendarTruncator: Now also truncate week