Convert shell commands to use child_process.spawn instead of exec

[jakecraige]

This will allow us to stream the output of the commands through rather than delaying it till the command finishes. It's just better UX really

[jakecraige]

We're seeing some issues post iOS8 with the buffer overflowing from output. A simple fix is to raise the amount allocated, though getting this in would future proof it forever

edit: The overflow should be fixed with the 0.0.13 release