Replace child_process.exec with child_process.spawn (fixes #16)

Under Windows it may happen that the process id of the spawned `node`-process will be reassigned. In the current implementation it's not possible to kill the process using this pid. *(It may also happen that another process with this pid will get killed!)*

To kill the process, `child.kill()` will now be used, which will require `child_process.spawn` instead of `child_process.exec`. 

`child_process.spawn` has a different syntax than `child_process.exec` and because of this I implemented a new `arg`-parameter in both the `server`- and the `main`-script.