Restructured django_polymorphic into a regular add-on application.

This is needed for the management commands, and also seems
to be a generally good idea for future viablity as well.
Also misc documentation updates.

dbreset

@@ -0,0 +1,5 @@
+#!/bin/bash
+
+rm /var/tmp/django-polymorphic-test-db.sqlite3
+./manage.py syncdb
+