SIGTERM took too long to shutdown

If ran on slow machine (e.g. small VPS) and resque-scheduler is managed by daemon manager (e.g. Supervisord),

during restart (e.g. */etc/init.d/supervisord restart*),

*run()* responded to SIGTERM but took too long to shutdown, resulting in multiple processes running.

This commit will exit the loop immediately when *@shutdown == true*. I think it is safe to do it this way since all the scheduled jobs themselves are documented inside *scheduled_jobs.yml*.

Sys-admins or deployers can simply make sure they don't deploy during the time described in *scheduled_jobs.yml*

This behavior is similar to resque where it simply perform *exit()* and kill all children without mercy.