Integration tests.

Note: child_process.execFile is not quite what I thought it was. The name of this repo/package probably should be changed.

execFile is not a special case for invoking another node process. Rather, it is pretty much the same as exec, in that the file argument has to be executable. Maybe the main difference is that arguments don't need to be quoted/escaped because they are passed in an array, instead of combined on a single command string.