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
Merge pull request #170 from alphagov/anonymise_ip_for_analytics

Anonymise IP tracking in Google Analytics