QTI 2.1 validated with num type exercises

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/exams@639 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
added string type exercises to exams2qti21()

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/exams@640 edb9625f-4e0d-4859-8d74-9fd3b1da38cb