Support for django 1.7

* django-nose version updated to 1.2 (instead of forked version used previously)
* Called setup() before executing the tests (https://docs.djangoproject.com/en/dev/releases/1.7/#standalone-scripts)
* Running migrate instead of syncdb in case of django version > 1.7
Well Django does do some metaclass magic when the class is defined, so I wouldn't be too surprised that it had some internal "these are the models that have been registered" thing that wasn't cleared between tests.

I'd start looking at the Model metaclass for that.