add _gat._anonymizeIp to google analytics config

This settings is to enable google analytics to anonymize IPs.

https://developers.google.com/analytics/devguides/collection/gajs/methods/gaJSApi_gat#_gat._anonymizelp
@ingorichter I don't know GA that well, but it seems like this might have to come _before_ the `_trackPageview` call in order to have the desired effect...