Add check support for handle: false

I asked on the sensu mailing list the other day if there was a supported way to have a check never send a notification to a handler, and Sean Porter suggested using handle: false. Looking through the sensu-chef code, I noticed that this wasn't yet supported, so I decided to add support. It makes sense to have this as a check-level option rather than using the "additional" hash, as it is a core directive and will disable handlers, which is another check-level option.

I would like to contribute back to sensu as much as I can, so criticisms and suggestions as I add support is much appreciated.