Return child process

[tiangolo]

Return the NodeJS child process from the invocation of getActiveWindow().

Currently, if getActiveWindow() is called with a repeats equal to -1 (infinity), there's no way to finish the process at some point. So, the process will keep "dangling".

With this change, the subprocess could be retrieved and manually killed as in:

var monitorProcess = monitor.getActiveWindow(callback, -1);
monitorProcess.kill();

I also updated the README to explain the parameters and the return of the ChildProcess.

[tiangolo]

BTW, the code is based on my previous PR, #21 . So that should go first.

[tiangolo]

I'm no longer working on this. I'm cleaning up my forked repos, I just wanted to leave this note before removing this branch and my fork. :nerd_face: :coffee: