AuthMemCookie: Support for authentication sources.

Update issue 215
This change sort of fixes the use of initSSO.php for
www/authmemcookie.php by making it possible to use authsources with
AuthMemCookie.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1817 44740490-163a-0410-bde0-09ae8108e29a