WIP: Integrating django-secure

For background info, please see the [django-developers thread](https://groups.google.com/d/topic/django-developers/t8ybImtdnpM/discussion).
I think the messages and the docs need to reflect what you actually set, or we're asking for confusion. I agree it's verbose; that's the downside of switching to dictified settings. I don't feel strongly either way about settings in a dict (the partial-update issue is another thing that makes me slightly more negative on that approach), but if we're doing them in a dict, we need to just find the most readable wording we can ('the HSTS_INCLUDE_SUBDOMAINS` key of your `SECURITY_MIDDLEWARE_CONFIG` setting`?) that is accurate and live with the verbosity.