Test multiple Django and Python versions on Travis ci; also added tox support

This tests Django 1.3, 1.4 and 1.5 and Python 2.6 and 2.7. See https://travis-ci.org/mbraak/django-fiber

Also added Tox support (http://codespeak.net/tox/) to test the same configurations as travis-ci.

Run this outside a virtualenv:

```
pip install tox==1.4.2
cd testproject
tox
```