Initial stab at monitoring_check::cluster

initial stab at cluster check, just need to know if i'm going in right direction?
This is a pretty tricky situation. I'm not really convinced it belongs in sensu-core. I don't think it can really live in the sensu client either.

All I know is, I want to distribute this check, fake the source, and have the "results" still look like it was only spawned from 1 server with an event rate that equals the event rate of a single server. (Not n times the number of sensu servers)