Fix #140: Add support for offline development via auto-login backend.

So this code seems to use "is `BROWSERID_AUTOLOGIN_EMAIL` set?" as the impetus to do various things. But in the documentation, you have to set this setting AND also add the authentication backend. Given that this has security implications, I'm inclined to either spruce this check up so it checks for both settings OR fix the docs so it's really clear that the helper keys off of just the `BROWSERID_AUTOLOGIN_EMAIL` setting regardless of what the AUTHENTICATION_BACKEND is set to.

Either that or I'm misreading and misunderstanding what's going on.