Mattdrayer/api coursegrades

@martynjames, @chrisndodge, initial version of the grades API.  The changeset provides us with the ability to submit (post) a grade for a particular piece of course content to all of the users in a given workgroup.  Additionally we also support the querying of grade information for a given course, with the ability to filter the grade set down to a specific piece of course content or user(s), or even both.

Take a look and let me know what you think -- will make any necessary updates/revisions tomorrow.
We're actually providing two distinct averages here.  The first average is for the requested queryset, which -- if user identifiers are provided -- is limited in scope to the upper bound setting.  The second average is for all students in the course (see 'course_grade_avg' below).  This value derives from a separate queryset vs. the first one.