correct shell commands under Windows for tex2image() added

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/exams@647 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
exams2qti21 officially added

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/exams@648 edb9625f-4e0d-4859-8d74-9fd3b1da38cb