Improved testing documentation

Thats a small fix, but it seems testing has a lot of room to improve.

Since this is a package intended only to use with django seems more consistent if the tests are written with django test case, instead of using nose.

I like nose a lot, even i have made some work for it https://github.com/jbalogh/django-nose/commits/master but really seems more useful and easy to maintain the test suite if is written using the django approach to testing.

I can make that switch, seems it will only take a couple hours, but i need to make sure this work is likely to be merged or not, besides after this i can make a better travis integration and tackle down some issues.