Send a ClosedEvent when SbtClient closes.

Previously we had a way to be notified on connector close
but not on SbtClient close, but the natural way to organize
code is to pass just the client to certain modules or objects,
we don't want every piece of code to have to be aware of the
connector. Also it's convenient to just get this in your
event handler and not "out of band."
Merge pull request #132 from sbt/wip/havocp-closed-event

Send a ClosedEvent when SbtClient closes.