tox: Add tox.ini file

tox: Add tox.ini file

Currently this contains calls to execute the following on the code
base:

* Unit tests (for all currently supported versions of Django). This
  requires the addition of a "test" 'local_settings' file
* PEP8 (or, rather, flake8)
* PyLint
* Coverage (based on unit tests)

These are designed in such a way that it should be possible to easily
add additional environment for testing (like Python3 or Django 1.8).

Signed-off-by: Stephen Finucane <stephenfinucane@hotmail.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>