Update version number for the 3.3.2 release.
Fix interactive behaviour scoring with a blank input.

We were evaluating PRTs even when they were not evaluatable, and that
was giving the wrong score.
Bump the version number for the 3.3.3 release.
Add support for statistics packages.
Increment version numbers.
Merge remote-tracking branch 'remotes/origin/master' into wipcasblocks

Conflicts:
	stack/cas/casstring.class.php
	stack/maxima/stackmaxima.mac
	version.php

This adds support for statistics functions.