Merged branch marks-fix. Fixes Google Code issue 144.
bin/ivle-marks: Completely rewritten. (Was hopelessly out-of-date, and didn't work at all. Now works on the current database model, and has new features to boot - able to select the semester and cutoff date).
ivle.worksheet.utils: Added calculate_mark, which is from the duplicated code in both bin/ivle-marks and ivle.webapp.tutorial.OfferingView.populate. Also added some extra optional arguments for calculating exercise/worksheet scores from a particular date.
ivle.webapp.tutorial: Call ivle.worksheet.utils.calculate_mark instead of manually calculating (avoid code duplication).
ivle.database: A few new methods on the Subject class, for retrieving offerings.