add pluginfile parameter so that supplements can be encoded with moodle's 
pluginfile function or as data URIs. this also removes the base64 switch 
in exams2moodle.



git-svn-id: svn://svn.r-forge.r-project.org/svnroot/exams@634 edb9625f-4e0d-4859-8d74-9fd3b1da38cb