Add Google Analytics key

[jefftriplett]

This code will live in includes/analytics.html.

[jefftriplett]

@sarahetoms I'd do this one sooner than some of the others. That way we don't forget when you get a surge of traffic.

@sarahetoms Can you set this up by the end of September?

[joelee484]

@jefftriplett -- I created this analytics code, tracking ID - UA-126219516-1. Can you add this to our site?