Moved the cluster plugin from cloud9 and adapted it slightly

[zefhemel]

to be more generic.

I think we can have the policy of putting plug-ins that wrap node.js builtin APIs in the architect main repo, and all others in separate repos.

@creationix or @sergi or @cadorn

[creationix]

I thought we wanted to get away from having multiple node modules in a single github repo. (like I've done with c9/vfs).

[zefhemel]

I don't think we really have this on npm, just use it like architect.log: "architect/plugins/architect.cluster"