Anonymise IP tracking in Google Analytics

Give further privacy to our users.

By calling this function, Google anonymise the IP (by removing the last octet)
before they write anything to disk.

http://support.google.com/analytics/bin/answer.py?hl=en&answer=2763052

I "tested" the change by looking for the parameter that the Google support page says should  be added to the request for ``_utm.gif``.