exit when child has exited, not earlier (mh)

With this little addition, the supervisor won't exit until the child process has really exited.

I need this feature because my app has a graceful shutdown.

Already tested it on my machine and works well. Also added few missing semicolons.
Yeah, I know - sorry for letting this sit around. I would *love* to have tests for this package, but just haven't yet figured out a time-efficient way to set some up and have them be useful.

For now, let's please remove these lines, and I'll be happy to merge it. Then, if people encounter problems, we can come back to this with a better understanding of why it might be necessary.