Remove NowListeningEvent, Stopped, NeedRebootEvent

I believe these were leftovers from requirements of the old way
sbt-remote-control worked. NeedRebootEvent never happens
anymore. NowListeningEvent and Stopped are replaced by the
onConnect / onError callbacks on SbtClient.
Merge pull request #101 from sbt/wip/havocp-drop-boot-events

Remove NowListeningEvent, Stopped, NeedRebootEvent