feature #392, authentication keys, album notification

* On album notification (for a group), sends one distinct email for each user
  with a new authentication key.

* When someone clicks the link with auth=<key> in URL, if the user is not
  already connected, Piwigo will automatically connect the user.

include/user.inc.php

@@ -65,6 +65,12 @@ if ($conf['apache_authentication'])
   }
 }
 
+// automatic login by authentication key
+if (isset($_GET['auth']))
+{
+  auth_key_login($_GET['auth']);
+}
+
 $user = build_user( $user['id'],
           ( defined('IN_ADMIN') and IN_ADMIN ) ? false : true // use cache ?
          );