Add support for multiple-domains with setDomainName

Just added a simple setter on the configuration.
Details on tracking multiple domains:
https://developers.google.com/analytics/devguides/collection/gajs/gaTrackingSite

But the reason I needed it was to do some testing on localhost:
http://stackoverflow.com/questions/4375447/can-you-test-google-analytics-on-a-localhost-address