Added course enrollment

Change-Id: I973fcb2672b53e0aa64158bea46b8cbb9c371c17

@rocha 
Okay. Here are examples of the two. This distinction is already made in the API. I would prefer not to adjust it for the LMS which won't necessarily be a consumer of the API once the dashboard is online. If it is confusing, we can explain it in the documentation with no mention of `OpaqueKey`.

`course_id = 123`
`course_key = 'edX/DemoX/Demo_Course'`