add support for ExitedExecutor message
add support for ExitedExecutor message

jrslv/mesos-go,mesosphere/mesos-go,basho-labs/mesos-go,tnachen/mesos-go,tnachen/go-mesos,ilackarms/mesos-go,jdef/mesos-go,oxtoacart/mesos-go,drewkerrigan/mesos-go,chrisjs/mesos-go,mesos/mesos-go