add basic btc button

index.html

@@ -75,6 +75,10 @@
     <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! #2048game" 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>
+      <span class="btc-donate">
+        <img src="meta/icon_bitcoin.png">Donate
+        <span class="address"><code>1Ec6onfsQmoP9kkL3zkpB6c5sA4PVcXU2i</code></span>
+      </span>
     </div>
   </div>