add extra setting SSLIFY_ADMIN_DISABLE to allow disable sslify_admin whe...

instead of relaying on settings.DEBUG, I think it's better to use a dedicated settings, with name matching django-sslify one. If this settings is not present, default value is DEBUG. 

I found it useful when I templating 403, 404, 500 files who need DEBUG=False to be viewed on a dev version. DEBUG=False enable django-sslify-admin, and it's not working with django runserver.