Update setup.py to handle -e in requirements.txt

Added setup.py to make packages installable

Use pure XBlock instead XModule.

Remove django-rest-framework

Adjust types and limits for common_grading/models.py

* Reduced MAX_LENGTH to 255 for varchar fields because that's the
  largest size with which we're guaranteed to be able to make an
  index on InnoDB with UTF-8 encoded text.
* Increased ESSAY_BODY_MAX_LENGTH to 10000 in response to limits
  discussion thread.
* Changed a couple of fields to text from varchar, just in case
  we ever decide to increase the size limits on them in the future.

We're not aiming to have an REST API in the MVP. Can re-add when
we get around to implementing it.

Create the service stub for peer grading

Updating the readme file

This stub just has some doc and empty functions so we can discuss if the contract is reasonable, then potentially build on this frame.

Cleaning up some of my initial attempts at models.

Deleting a few files I don't believe we'll need.

Added nose and lettuce test infrastructure