Define create_user_if_not_exists option as true by default

[ftassi]

FacebookFactory defines create_user_if_not_exists as false and the FacebookProvider implementation in the docs has a loadUserByUsername which create a new user if this does not exists and ignore the create_user_if_not_exists options.

What this PR does:

1. Define create_user_if_not_exists true by default
2. Add to the documentation a FacebookProvider that creates the user only if create_user_if_not_exists is true.

Users can now switch creation of new users by altering the configuration params and the FacebookProvider implementation looks more obvious.

Feedback ?