Added delta tracking to documents.

All saves on exisiting items do set / unset operations only on changed fields.
* Note lists and dicts generally do set operations for things like pop() del[key]
  As there is no easy map to unset and explicitly matches the new list / dict

fixes #18

tests/django_tests.py

@@ -1,4 +1,3 @@
-
 # -*- coding: utf-8 -*-
 
 import unittest