Allow a specific signal to be sent when killing a process.

Some processes (Hi, webrick!) don't honor SIGTERM and return errors on SIGKILL, which can make processes relying on error codes, um... error.

This change allows you to pass a signal to #stop, which it will attempt before trying SIGTERM and SIGKILL.  The parameter is optional, but can be called like ```child_process.stop(3, "INT")```.

Also, added a change to the TERM escalation test -- A sleep so it has time to get setup before running.