Provide ENV variable for enabling Google Analytics tracking

[mikefowler]

By default, no GA script would be injected, but if an ENV variable was set (i.e. GA_TRACKING_ID) then the proper script block would appear in the page with the tracking ID set.

[jonmagic]

Love this idea. Might do the same for Gauges too, my preferred analytics service :)

[jonmagic]

I forgot to do this before the emails went out this morning so I just quickly threw in a gauges tracking script and sent @mikefowler an invite.