cluster: emit 'message' event on cluster master

For consistency with the worker 'exit', 'online', 'disconnect', and
'listening' events which are emitted on worker and cluster, also emit
'message' on cluster.

Reviewed-by: Sam Roberts <vieuxtech@gmail.com>
Reviewed-by: Christian Tellnes <christian@tellnes.no>
Reviewed-by: Stephen Belanger <admin@stephenbelanger.com>
PR-URL: https://github.com/nodejs/io.js/pull/861