Supporting per-request overrides

Hi,

I needed to SSLify a whole app deployed on Heroku, but then hit a problem with an external web hook that didn't support SSL. That's why I added support for passing a list of callables via the SSLIFY_DISABLE_FOR_REQUEST setting that get evaluated for every request. My current project now uses a simple `startswith` check for the request path to disable SSL for the hook URL in question.

I guess this may come in handy for other people. Of course, I'd be happy to change/fix stuff to this PR.

My current project is an old Django 1.4.x app and I had to change some portions of the test project to be able to run the tests.