Implemented the Mappers class.  Added mappers to create arraylists of students, teachers, courses, etc, from generic resultsets of personids, courseids, or gradeitemids.  These resultsets will be created by calling the specific query methods in DBConnector.  See Tester for example (list all courses a student is in).

Cleared up the ID settings in all models so that it's specific (eg courseid, not course.id or id).  Changed queries to select id as e.g. courseid so it's always the same.