Update to Google Universal Analytics

This is an optimized version of the official Google Universal Analytics snippet,
based on http://mathiasbynens.be/notes/async-analytics-snippet.
The only difference is that this version still uses
`document.getElementsByTagName('script')[0]` instead of `document.scripts[0]`
for Firefox < 9 support.

Unminified, optimized code:

    (function(window, document, script, variableName, scriptElement, firstScript) {
      window['GoogleAnalyticsObject'] = variableName;
      window[variableName] || (window[variableName] = function() {
        (window[variableName].q = window[variableName].q || []).push(arguments)
      });
      window[variableName].l = +new Date;
      scriptElement = document.createElement(script);
      firstScript = document.getElementsByTagName(script)[0];
      scriptElement.src = '//www.google-analytics.com/analytics.js';
      firstScript.parentNode.insertBefore(scriptElement, firstScript)
    }(window, document, 'script', 'ga'));

    ga('create', 'UA-XXXX-Y');
    ga('send', 'pageview');

Minified:

    (function(b,o,i,l,e,r){b.GoogleAnalyticsObject=l;b[l]||(b[l]=function(){(b[l].q=b[l].q||[]).push(arguments)});b[l].l=+new Date;e=o.createElement(i);r=o.getElementsByTagName(i)[0];e.src='//www.google-analytics.com/analytics.js';r.parentNode.insertBefore(e,r)}(window,document,'script','ga'));ga('create','UA-XXXX-Y');ga('send','pageview');

Closes #1347 and #1369.