Refactor courses.course_id to courses.course_name

This change *will* break things, as I've changed the database schema slightly. 

I renamed `db.courses.course_id` to `db.courses.course_name`. This is to help differentiate it from the field `db.auth_user.course_id`, which is actually a reference to `db.courses.id` (not the previously-named `db.courses.course_id`).

When this is deployed to Webfaction, the tables currently in the Postgres database will have be dropped (will also have to delete everything in the runestone/databases folder to force web2py to recreate all tables).

This pull also fixes issue #156. If you'd rather not make this refactor now, I can split the fix for issue #156 into a separate pull request