Asynchronous Twitter Tweet Button

gistfile1.js

<script type="text/javascript">
  (function() {
    var t = document.createElement("script");
    t.type = "text/javascript";
    t.src = "//platform.twitter.com/widgets.js";
    t.async = true;
    document.getElementsByTagName('head')[0].appendChild(t);
  })();
</script>
<a href="https://twitter.com/share" class="twitter-share-button" data-count="horizontal">Tweet</a>