acu124520 - setting shutdown hook only once

it is possible for @tick_queue_running to be set
back to false if the tick_queue is empty when a
tick got completed. Hence we need to make sure
that a shutdown hook is added every time the 
tick_queue is empty.
Merge pull request #7 from rightscale/cornsilk_13_13_acu124520__dme_dane_djr__setting_add_shutdown_hook_only_once

Cornsilk 13 13 acu124520  dme dane djr  setting add shutdown hook only once