Fix FANDJANGO_SITE_URL settings attribute

* `FANDJANGO_SITE_URL` is no longer mandatory.
* When using `FacebookWebMiddleware` if `FANDJANGO_SITE_URL` is not set, the post auth redirect url will be constructed automatically.

Documentation PR: https://github.com/jgorset/fandjango/pull/100

:tada::cocktail::star2: Happy New Year! :star2::cocktail::tada: