save NextToken from ListDomains requests if applicable

We realized that we had more than the server-imposed limit of 100 ListDomains results when we tried to check if we already have a domain or not, which then led us to realize that php_sdb2's [listDomains() function](https://github.com/g-g/php-sdb2/blob/master/src/g_g/php_sdb2/SimpleDB.php#L232) doesn't save the NextToken value in ListDomainsResult. I think this patch should allow users to page through more than 100 domains.