Make strong-cli catch signals and deliver to child

Fix #SLN-1056
Fix #140
These conditions are mutually exclusive:

"f the process terminated normally, code is the final exit code of the process, otherwise null. If the process terminated due to receipt of a signal, signal is the string name of the signal, otherwise null."   - http://nodejs.org/api/child_process.html#child_process_event_exit

So, conditional is equivalent to `if (lastSignal)`.