Publish watchHost as started after it has called StreamEvents

This fixes a race where the tests believe watchHost is ready to stream
events so send it some events, but in fact the watchHost goroutine has
not yet called StreamEvents, so the event from the tests goes nowhere,
and watchHost just hangs once it actually starts waiting for events.

Signed-off-by: Lewis Marshall <lewis@lmars.net>
Merge pull request #37 from flynn/fix-watch-host-race

Publish watchHost as started after it has called StreamEvents