Implement rate-limiting

http://www.vinaysahni.com/best-practices-for-a-pragmatic-restful-api#rate-limiting
`empty($this->account->roles` is an early return for error check on [L#177](https://github.com/Gizra/restful/pull/71/files#diff-55e7308550589d57aff8649137fc29ecR177).