Fixed south and empty responses from middleware

# Empty response

I didn't saw any issue open about this topic, but it happened to me lots of time.

How it works:

On `django_facebook/middleware.py:12` it creates a `ScriptRedirect` instance to be used when a login is required from facebook, but if you are using gunicorn or any other process-persistent application server, after the first request, Django will consume the response and leave it there, but on the second request it will not "reset" the content and serve it again, instead it will just response an empty string and all you will see is a blank page.

# South

The quick and dirty solution to south was completely remove it from the code, so no one will have trouble again with custom user models or old user models. 

This is not the best solution but is the only one I have to keep using Django-facebook.