Add Universal Analytics (bug 1018934)

~~We need to add some ability to merge settings with settings provided via the page like we currently do in webpay. That can likely be done as a separate thing though.~~ See: #73

Based on  https://github.com/mozilla/fireplace/blob/master/hearth/media/js/tracking.js this is mostly an UA version of the same code we have now on webpay.