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. 
nitpick, but I prefer something like:
```python
    (response, content) = _post_patched_request(
        body, client, url,
        method, content_type
    )
```
or
```python
   (response, content) = _post_patched_request(
        body,
        client,
        url,
        method,
        content_type
    )
```