Implement rate-limiting

http://www.vinaysahni.com/best-practices-for-a-pragmatic-restful-api#rate-limiting
I was thinking of `RestfulRateLimitRequest` as `<ModuelNamespace><PluginType><EventName>`. I'd rather see

```php
class RestfulRateLimitPosting implements RestfulRateLimitInterface { … }
```
than
```php
class RestfulRateLimitPosting extends RestfulRateLimitEventBase { … }
```

Since `RestfulRateLimitEventBase` as simple as it would almost be duplicating the interface.