Handle QUIT signal

After required this library, I found that resque worker doesn't handle QUIT signal properly.

https://github.com/defunkt/resque/blob/1-x-stable/README.markdown#signals

You can compare the behaviour with sending QUIT to a process w/ resque-multi-job-forks and w/o resque-multi-job-forks.

The child worker needs to check `@shutdown` and release_fork when it is true.