Added method remove_role_from_user_for_resource. remove_role method doesn't work correctly.

When using the "remove_role" method to remove a role from a user for a particular resource, it removes the role itself from the roles table, thus, removing the role from all users who have that role instead of only the user we called "remove_role" on. A "remove_role_from_user_for_resource" method was added that fixes this problem.