Graceful TERM (for 1-x-stable)

Add graceful_term option so TERM works like QUIT
    
If Worker#graceful_term is enabled, SIGTERM waits for a job to complete before exiting (just like SIGQUIT).

Worker#graceful_term defaults to ENV['GRACEFUL_TERM'].

This makes it possible to use resque with runit/daemontools without resorting to a wrapper script.