Add support for graceful shutdown via TERM 

See http://hone.heroku.com/resque/2012/08/21/resque-signals.html

It's just a matter of passing down the ENV variable for TERM_CHILD to a property on the Worker.  Foreman, and suchlike, can then just send SIGTERM to the pool master, which will ultimately raise a SignalException in the worker children, allowing graceful handling of shutdown.