Added course enrollment

Change-Id: I973fcb2672b53e0aa64158bea46b8cbb9c371c17

@rocha 
You are not going to be the only person working on this code.

The conventions of `course_key` and `course_id` are well established in the LMS. Changing them here just make things confusing for another person coming or using the code. For example, an LMS user may pass a `CourseKey` instance to the constructor above.