added support for ip address anonymization

I added a new option 

    :anonymizeIp => true

for ip address anonymization. This is useful for sites hosted in Germany in order to meet legal requirements as described under #4 in the following document (in German):

http://www.datenschutz-hamburg.de/uploads/media/GoogleAnalytics_Hinweise_fuer_Webseitenbetreiber_in_Hamburg_01.pdf

see also the api docs: 

https://developers.google.com/analytics/devguides/collection/gajs/methods/gaJSApi_gat?hl=de#_gat._anonymizeIp