Added course enrollment

Change-Id: I973fcb2672b53e0aa64158bea46b8cbb9c371c17

@rocha 
Mmm. Why did you remove the `unicode` call? Will `self.client.get` do the proper encoding. We already have courses that require unicode (chinese). However, calling `unicode` here may not be the correct thing to do for those cases. Maybe what we need here is something like `urllib.quote(self.course_key.encode('utf-8'))` ?

We should write a test for that, and also on the API side.