Implement rate-limiting

http://www.vinaysahni.com/best-practices-for-a-pragmatic-restful-api#rate-limiting
I don't see the real value on that. `RestfulBaseEntity::setHttpHeader()` will only store key-values in an array to ultimately use `drupal_add_http_header()` outside the scope of the class (in restful.module). So the only thing we would be reusing is the storage array.

> Disadvantage: More coupled with the Restful handler, although I can't think of a use case where the rate limit would be by itself.

But we may want the rate limit to apply to different restful handlers without having to maintain one inner restful plugin.