Ensure *nix kills entire process tree.

Fixes #333 and fixes #335. Amazingly Windows support always had the full process tree being killed, but this solves it for mac and linux now. My only grip is we have a new dependancy for the project, but I don't really think that's such a big deal.
Ensure only subtree of child is killed

Otherwise when nodemon is required in, it could accidently kill children of the parent process that nodemon shouldn't really be touching.
Tests for subtree nukage.
Test failed because "debug" => "Debug"
Merge pull request #338 from remy/fix/kill-subtree

Ensure *nix kills entire process tree.