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.
Just to confirm, this just caps the # of allowable user_ids to query against. But the average will be *all* students in the course, right?