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.
Well, to be honest, I cannot remember perfectly. This commit of mine is pretty old. Five months is a long time for a busy hacker. I have tested it with my own apps and had this deadlock issue: the child process crashed during it's own graceful shutdown procedure and made the parent process wait forever. Hence the `child.removeAllListeners('exit')` to stop this. Something like that. I reckon I should have written more comments. But on the other hand, if you had asked me that question earlier I could have given you a much better answer ...

Also, the lack of unit tests here is another disadvantage here!