Added course enrollment

Change-Id: I973fcb2672b53e0aa64158bea46b8cbb9c371c17

@rocha 
I discussed this with @cpennington back when I originally wrote this code.

We *want* people to pass in CourseKey objects here. unicode() will do the right thing regardless of whether they do or not though (a string is fine too).

Are you planning on referring to courses by the same IDs that the LMS uses? If so we should just call Unicode() not to_deprecated_string(). It should do the right thing.

Cale can weigh in here if I misunderstood.