Prevent Invitation istanced with no arguments from doing anything

darcs-hash:20090312102112-58103-d21d3ede0a1dcadeac908d192aa68e3a3411c620
since GreenEngine.stop now blocks until it's stopped, sleeping before exiting is unnecessary

darcs-hash:20090312121009-b2873-9470c9e2111e6582fecb78bca424d13fa5f783fb