Ziafazal/course module completions

Course module completion implementation to support SLNS-62
@mattdrayer please review. Thanks
Rather than simply exposing this resource directly, I think it's important to consider the context in which a completion is being recorded and present a URI that maps more closely to this event.  In this particular case, a User has accomplished some (unknown to us) task, and the client has determined that the course content should now be considered completed.

So, we might instead present a URI such as /courses/(course_id)/content/(content_id)/completions, and simply take a user_id as the POST payload.  This would additionally allow us to GET the list of completions for a particular piece of course content.  

Alternatively, we could present the more generic URI of /courses/(course_id)/completions, which would return the entire set of completed modules for the specified course, still supporting the available filtering by content and user.  This is probably a better way to go because it maps more closely to the current implementation.