Updated setup.py
Merge pull request #306 from edx/will/update-setup-py

Updated setup.py
Add AI grading architecture documentation.
Merge pull request #305 from edx/will/machine-grading-arch

Add AI grading architecture documentation.
Make peer assessment an optional step.

This also creates the AssessmentWorkflowStep model to track all
steps for new workflows. Some problems with it now:

* a lot of broken tests
* some display oddities on the peer step during (peer, self) workflows
* have not tested to see if there are concurrency issues with step
  model creation under load.
Updating tests and some simple cleanup.