Turns out you have to let the database know that a value can be None,
who knew? Migration included.

To fix the buggy behavior reported in TNL-3880, we need to ensure
that the submission we're dropping is unlinked from the student_item
used to find it. This prevents code (say, an ORA staff tool) from
constructing a student item with an id, a course, and a problem, and
using that to find a "cleared" submission.

Update the DRF version requirement to match edx-platform

Add new app_label to ScoreAnnotation.

Django upgrade/1.8 release

Conflicts:
	setup.py
	submissions/models.py

We had made the migrations from master, but that isn't what edx-platform
was using yet.  This migration is re-made from 0.1.0, which removes
things that happened after 0.1.0.

remove deprecation warnings

Update the submission version.

Add the ability to annotate scores.

Upgrade djangorestframework to v3.1

update django-nose

* Upgrade DRF to v3.1
* Use JSONField on the model instead of custom serializer field for the submission answer.

Upgrade django-model-utils to 2.3.1

Update pytz to reflect upstream version change

In certain circumstances the version of Pytz in this file will conflict with versions of pytz required by edx-platform in `requirements/edx/base.txt`. This PR makes the version in this repository match.

Upgrade django-extensions to 1.5.5

Add get_all_submissions method to the API

This is consistent with the default ordering of the model and avoids ties in
case to items have the same timestamp.

Adding signals for scoring events.