course and enrolment models

this depends on #19

1. new course model

2. course user relation model: enrolment
Yup :)

Order of columns indeed does not affect performance. but when you're looking at a database schema using your database admin tool you want columns which are related to be grouped together. Rails might not be the only application using the database.

It's not whether users are queried more often. It depends on the generated SQL -- if the SQL generated is course then user (which I believe it is, because we find Course first) then the index must be course then user... You need to check this.