Added Piwik tracking code (to quickly know about new links to my site)

My tracking policy:
- The DoNotTrack header is respected. If you don't want to be tracked, then set this header.
- You will not be tracked when javascript is disabled, no noscript tag is used.
- The last two bytes of your ip address will be set to 0, you cannot be uniquely identified.

I even highly recommend using the DoNotTrack header and a plugin like NoScript.