add twitter sharing button

index.html

@@ -61,6 +61,10 @@
     <p>
       Created by <a href="http://gabrielecirulli.com" target="_blank">Gabriele Cirulli.</a> Based on <a href="https://itunes.apple.com/us/app/1024!/id823499224" target="_blank">1024 by Veewo Studio.</a>
     </p>
+    <div class="sharing">
+      <a href="https://twitter.com/share" class="twitter-share-button" data-text="Check out 2048, a game where you join numbers to score high!" data-via="gabrielecirulli">Tweet</a>
+      <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>
+    </div>
   </div>
   <script>
     (function(i,s,o,g,r,a,m){i["GoogleAnalyticsObject"]=r;i[r]=i[r]||function(){

style/main.css

@@ -393,3 +393,7 @@ hr {
 
 .game-explanation {
   margin-top: 50px; }
+
+.sharing {
+  margin-top: 20px;
+  text-align: center; }

style/main.scss

@@ -364,3 +364,8 @@ hr {
 .game-explanation {
   margin-top: 50px;
 }
+
+.sharing {
+  margin-top: 20px;
+  text-align: center;
+}