Properly synchronised tox and travis test config

This does a number of things:

* Fixes tox.ini to not run Django 1.4 tests (as its not supported any more)

* Fixes tox.ini to run Python 3.3 tests and Python 2.6

* Use "./runtests.py" for running travis tests, not something different,
  so everything is consistent. One Way To Do It.

* Removes requirements.txt, as this is confusing and not needed
  (you can use "setup.py develop" for installing in development)

This fixes several bugs with Travis tests:

* They ran against different (and underspecified) versions of packages from tox.ini

* They didn't run tests fully - Travis didn't run the attachments tests on
  Django < 1.6.  This is the kind of thing that is easily overlooked if
  you've got multiple ways to run tests.
Merge pull request #358 from spookylukey/synchronise_travis_and_tox_2

TOOWTDI principle: Make Travis test configuration link to tox configuration and use tox as test setup suite.