Fix saving sessions in PHP 5.4 with user session handlers (fix #12)

When session storage functions are set with session_set_save_handler()
(this is the "mod_user" mode), mod_data will be NULL in PHP 5.4, and
suhosin session hooks will bail out.
PHP 5.4 allows to check this with mod_user_implemented instead.
Merge pull request #2 from blino/master

Fix saving sessions in PHP 5.4 with user session handlers (fix #12)