Add proper testing to the project

I've started adding tests for this project. On the way I have a few questions:

- what Django versions do you want to support?
- what Python versions do you want to support?

I have also integrated project with travis: ![django-cors-headers](https://api.travis-ci.org/lukaszb/django-cors-headers.png?branch=tests)

Am going to finish test suite before this would be merged. Just let me know if you're interested in those changes at all.