Fix missing os._exit() argument

The os._exit() function takes an argument that was left out; this adds
the appropriate argument.
Fix definition of --debug

The --debug option is supposed to be a boolean option, but it was
declared as an option taking an argument.  This corrects the issue.
Wait for the hub to exit

The start_hub() function must wait for all threads to finish before
it can exit.  This is accomplished by adding a call to gevent.wait().