Add NODE_ENV option

I tried to base this on the port option as much as possible.  Here's a
summary of what I've done.

- Added the NODE_ENV option with a default of undefined
- Updated README with option
- Added unit test in `custom_node_env_test.js` (modified the app.js to
  have a configure('production') piece)
- Fixed some jshint pathing that was missing files and fixed jshint
  errors it found.

Feedback welcome if you'd like something altered.
Update tests to always test environment
process.env is restored to original state when server is stopped
Fix bug where server.kill is called when server is in foreground
Fix server in foreground stopping immediately after running
Add express:custom_background for non-unit testing :)