Add internal Google Analytics template

This commits also adds the GoogleAnalytics
variable to insert the tracking code.

Fixes #1424.

docs/content/overview/introduction.md

@@ -127,6 +127,7 @@ Hugo boasts the following features:
 ### Additional Features
 
   * Integrated [Disqus](https://disqus.com/) comment support
+  * Integrated [Google Analytics](https://google-analytics.com/) support
   * Automatic [RSS](/layout/rss/) creation
   * Support for [Go](http://golang.org/pkg/html/template/), [Amber](https://github.com/eknkc/amber) and [Ace](http://ace.yoss.si/) HTML templates
   * Syntax [highlighting](/extras/highlighting/) powered by [Pygments](http://pygments.org/)
@@ -183,4 +184,3 @@ as I have writing it.
  * [Join the Mailing List](/community/mailing-list/)
  * [Star us on GitHub](https://github.com/spf13/hugo)
  * [Discussion Forum](http://discuss.gohugo.io/)
-

docs/content/templates/variables.md

@@ -127,6 +127,7 @@ Also available is `.Site` which has the following:
 **.Site.Author** A map of the authors as defined in the site configuration.<br>
 **.Site.LanguageCode** A string representing the language as defined in the site configuration.<br>
 **.Site.DisqusShortname** A string representing the shortname of the Disqus shortcode as defined in the site configuration.<br>
+**.Site.GoogleAnalytics** A string representing your tracking code for Google Analytics as defined in the site configuration.<br>
 **.Site.Copyright** A string representing the copyright of your web site as defined in the site configuration.<br>
 **.Site.LastChange** A string representing the date/time of the most recent change to your site, based on the [`date` variable]({{< ref "content/front-matter.md#required-variables" >}}) in the front matter of your content pages.<br>
 **.Site.Permalinks** A string to override the default permalink format. Defined in the site configuration.<br>