Add tox

This adds a series of tox environments for better multi-environment
testing.
Fix deprecated import

Django 1.4 moved things from django.conf.urls.defaults to
django.conf.urls. This updates the test_app accordingly.
Add note about tox to documentation
Fix MANFIEST.in

This fixes any test systems that install tidings before testing it.
Installing it would miss several files causing the tests to fail.
Fix for django 1.6.

`model._meta._fields()` is now `model._meta.fields`