Update Lib/FB.php

When not using 'Facebook.Connect' in $components var the self::$Facebook static variable is not set.
This cause a warning on call_user_func and result of a non working plugin.

Being new to cakephp I only see this fix for now. This fix issue #48
Merge pull request #118 from antoinegomez/patch-1

Update Lib/FB.php (quick fix for issue #48)