Add throttle method that does not raise exception

There are use cases where throttling is desired within a method without an exception bubbling up. The combination of `throttle!` and `throttled?` can be used to largely achieve the effect, but an exception will still be raised at the time the threshold is hit.

This will allow easy switching on whether or not the threshold has been reached without the fear of an undesired exception popping up.

@hsume2 @dadah89 @morten