Support disable overrides for specific requests.

Add a SSLIFY_DISABLE_FOR_REQUEST list of callables and evaluate them for
the current request. If any of them returns True, abort.

The global SSLIFY_DISABLE setting always takes precedence.
Django 1.4.x compatibility fix

- not using SessionAuthenticationMiddleware
- avoiding a "template not found error" for 404.html with a simple template
loader.
Document SSLIFY_DISABLE_FOR_REQUEST in the readme.