Add facebook_login_required

Add decorator to check if a user is logged in.

If he is, we check if this is the same user that is trying to access the app from facebook.

This is meant to be used with a canvas application, so for every page, the user needs to be logged in.

It's possible that 2 different users use facebook in the same computer and browser, so when the 2nd user access the app, django would recognize it as being logged in as the first, so we need to detect this, logout the 1st user and then login the new one.