READY FOR REVIEW: PyLTI edX LTI 2.0 support

This adds post_grade2(grade, user=) to the LTI object. Thus if enduser knows <anon_id> it can submit grade for that user. 
not for `user_id` since it is new to this branch, but for response_url it would.  That said how many are out there, and since we are still calling this pre 1.0 it is generally ok as long as you document it.  Maybe that is worth an issue and convert any other methods that should be properties?