Add support for ratelimiting via annotations

This allows adding rate limiting via annotations to controllers, as one example:

```
@UserRateThrottle(limit=5, period=100)
@AnonRateThrottle(limit=1, period=100)
```

Would mean that logged-in users can access the page 5 times within 100 seconds, and anonymous users 1 time within 100 seconds. If only an AnonRateThrottle is specified that one will also be applied to logged-in users.

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>