remove tables from db upon install

git-svn-id: https://plugins.svn.wordpress.org/an-gradebook/trunk@871488 b8457f37-d9ea-0310-8a92-e5e31aec5664
stagging v2.0.2

git-svn-id: https://plugins.svn.wordpress.org/an-gradebook/trunk@871489 b8457f37-d9ea-0310-8a92-e5e31aec5664