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. 
similar style nit:
perhaps:
```python
from .common import (
    LTI_SESSION_KEY, LTI_PROPERTY_LIST,
    LTI_ROLES, verify_request_common,
    post_message, post_message2, generate_request_xml,
    LTIException, LTIRoleException, LTINotInSessionException,
    LTIPostMessageException
)
```
or
```python
from .common import (
    LTI_SESSION_KEY,
    LTI_PROPERTY_LIST,
    LTI_ROLES,
    verify_request_common,
    post_message,
    post_message2,
    generate_request_xml,
    LTIException,
    LTIRoleException,
    LTINotInSessionException,
    LTIPostMessageException
)
```
would be easier to read