An improved Django integration with fixtures support

I've removed the `django-sane-testing` "dependency" in favor of [`django-nose`](http://github.com/jbalogh/django-nose) which uses default Django testing tools and plays well with other Django related tools.

I've also added a way to specify fixtures at a scenario level like via `scc.fixtures` using Django [code and behavior](http://docs.djangoproject.com/en/dev/topics/testing/#fixture-loading). For example : 

    @Before
    def before(sc):
        scc.fixtures = ['birds', 'mammals.json']

It makes it easier to use freshen with Django but may break old test that rely heavily on django-sane-testing fixtures.