Added Facebook mobile app/web based auth middleware.

This is a single middleware to allow Fandjango to work with:
* Facebook mobile apps
* Websites which want to offer Facebook login.

Usage:
* Add middleware `fandjango.middleware.FacebookWebMiddleware` to `MIDDLEWARE_CLASSES`
* Set `FANDJANGO_SITE_URL` to the url of your mobile app page or website.

Notes:
* I have modified the existing `FacebookMiddleware` slightly to ensure full backwards compatibility. The 2 middlewares can be run side by side, or independently.
* A cookie `oauth_token` is set to cache the user's authentication.
* Includes a comprehensive new suite of tests and mocking improvements to existing ones.
* I've been working on this for a few months, it's been running like a dream over at http://apps.facebook.com/betty-app.
* This is a beefy PR. I welcome your feedback/critique.