Use new TERM_CHILD logic for sending TERM signals to child worker proces...

...ses. Fixes #20.

There's new logic for how Resque handles signals:

* QUIT: sets a `shutdown` flag that triggers a `break` on the next worker loop
* TERM or KILL: sends TERM signal to active worker process, waits n seconds, then sends KILL

the old logic always sends a KILL signal to the active worker process.