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. 
:) funny thing.
edX does not have LTI 2 launch request, only grade handler. And grade handler is build like that:
https://github.com/starteam/edx-platform/blob/master/common/lib/xmodule/xmodule/lti_2_util.py

I'm just providing functionality currently available.

On the other note, at LTI consumer meeting today we talked about LTI 2.x support and while it is TBD I think we would evolve code as edX platform evolves.