Revert "Delete ga.js"

This reverts commit 743c7d1.

Author: talbring

_includes/ga.js

@@ -0,0 +1,6 @@
+(function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
+(i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
+m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
+})(window,document,'script','https://www.google-analytics.com/analytics.js','ga');
+ga('create', '{{ site.google_analytics }}', 'auto');
+ga('send', 'pageview');