basic roles

1. add roles to User and InstanceUser model

2. add two tests and refine existing tests, remove should statement ( http://betterspecs.org/#should )
Your migration is meant to be idempotent. And also when you set up a new Coursemology you are supposed to use `rake db:load` and not `rake db:migrate` so schema.rb is authoritative. Your migration just makes an old database schema conform to the new one.

So change it. Your database table should have a consistent and logical layout.