Implements new structure and adds disk information

I went and implemented the proposed structure as I think it is way more clear. :)

Next to that I added [node-df](https://www.npmjs.org/package/node-df) to get the disk space, but this only works on unix systems. I could work on implementing Windows support in the future if that is requested, but I won't need it myself. 

The values of os.type() and os.release() are added tot the system object too.

I'm not sure what you want to put in the value 'system.disk' , the total amount of storage, or total available storage or just from one volume or disk?