Run processes detached and kill the process group

[sashton]

Fix for https://github.com/jasongilman/proto-repl/issues/291

I've tested this on OSX, but I don't have access to Windows to verify it there.

Based on these recommendations:

• http://azimi.me/2014/12/31/kill-child_process-node-js.html
• https://github.com/nodejs/node-v0.x-archive/issues/1811

[jasongilman]

Thanks for this fix!