Now you can add existing students by ID, and add the same student to multiple gradebooks

git-svn-id: https://plugins.svn.wordpress.org/an-gradebook/trunk@872052 b8457f37-d9ea-0310-8a92-e5e31aec5664
stagging v2.0.6

git-svn-id: https://plugins.svn.wordpress.org/an-gradebook/trunk@872053 b8457f37-d9ea-0310-8a92-e5e31aec5664