Added course enrollment

Change-Id: I973fcb2672b53e0aa64158bea46b8cbb9c371c17

@rocha 
unicode(course_key) for an "old" course will return something like
'slashes:edx+demo100+2014_spring'.

We have no way (yet) to convert to that into the course_id (the string) in
the API server, so it matches what we are using in the pipeline. In other
words, the API uses the old `course_id`.

One solution here is that we can get the course_key and use
`to_deprecated_string` when calling the API.

I guess we should make the API support serialized `course_keys`? That was
the question we asked a while ago and never got an answer :(