Added course enrollment

Change-Id: I973fcb2672b53e0aa64158bea46b8cbb9c371c17

@rocha 
@clintonb if it's not too convoluted, we should mock out requests.get rather than introducing another library (httpretty). [Here's an example of how we're using it in the edx-platform repo.](https://github.com/edx/edx-platform/blob/master/common/lib/xmodule/xmodule/tests/test_video.py#L560)