Tox and travis for Django 1.3, 1.4 and 1.5

* Add tox config for Django 1.4 and 1.5. Django 1.3 tests are broken.
* Fix tests for Django 1.3.
* Don't hard code interpreter version in tox config.
* Update contributing docs to show how to run tox tests.
* Add Django 1.3 to travis builds.
* Add override settings dependency for Django 1.3 to travis builds.
* Add python 2.6 to tox config.
* Fix tox/travis so that they test the lastest release of each minor version.
* Add a note to contributing doc to install tox first and make example commands more consistant.
* Add note about backwards compatibility import.