feat(*) add twitter button

Put the script init part at the very end of the html,
so that it will execute after the app's scripts in production
(possible to optimize it more in a near future)

src/index.html

@@ -56,6 +56,9 @@ <h1>Welcome</h1>
         </ul>
       </ul>
       <p><a href="https://github.com/topheman/vanilla-es6-jspm/wiki" title="About vanilla-es6-jspm on github">Learn more on github ...</a></p>
+      <p>
+        <a href="https://twitter.com/share" class="twitter-share-button" data-url="https://topheman.github.io/vanilla-es6-jspm" data-text="Project boilerplate for #ES6 #Jspm based apps w/ full production workflow #gulp #continuousintegration" data-via="topheman" data-size="large" data-count="none"><span style="display: none;">Tweet</span></a>
+      </p>
     </div>
   </div>
 
@@ -84,5 +87,7 @@ <h1>Welcome</h1>
 <!-- endbuild -->
 <!-- inject:analytics -->
 <!-- endinject -->
+<!-- twitter share button init (added at the very end so that it won't block app scripts in production - @todo possible to optimize a little more ...) -->
+<script>!function(d,s,id){var js,fjs=d.getElementsByTagName(s)[0],p=/^http:/.test(d.location)?'http':'https';if(!d.getElementById(id)){js=d.createElement(s);js.id=id;js.src=p+'://platform.twitter.com/widgets.js';fjs.parentNode.insertBefore(js,fjs);}}(document, 'script', 'twitter-wjs');</script>
 </body>
 </html>