exec(git...) does not work in windows

@achambers 

Thanks for putting this package together.  So, basically the issue here is that you cannot do a git command with the exec() function.  For some reason it errors out.  So instead what works is using the child_process.spawn function.

I don't catch errors for now, but we should merge this in and if someone has a good way to catch the error that would be awesome.