Switch to use child_process.spawn

Spawn differs a little from exec in that it provides a stream from which we can continuously read form.

This makes for a more pleasant experience as we can see our child_process output as it's being created, making for a more real-time experience.

More info about spawn vs. exec: http://www.hacksparrow.com/difference-between-spawn-and-exec-of-node-js-child_process.html