Added code to disable GA

Signed-off-by: Martin Herbst <mail@mherbst.de>

_includes/header.html

@@ -17,7 +17,7 @@
   <!-- Font -->
   <link href='https://fonts.googleapis.com/css?family=Lato:300,400,700' rel='stylesheet' type='text/css' />
   <link rel="canonical" href="{{ site.url }}{{ page.url }}" />
-
+  <script type="text/javascript">var gaProperty = 'UA-47717934-3';var disableStr = 'ga-disable-' + gaProperty;if (document.cookie.indexOf(disableStr + '=true') > -1) {window[disableStr] = true;}</script>
   <script>
   (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),
@@ -67,4 +67,4 @@
           </div>
         </div>
       </nav>
-    </div>
+    </div>