Loads the Pinterest JavaScript asynchronously. I add this to my WordPress website with wp_head, but you can copy and paste this snippet anywhere on your page. You only need to load this script once per page, no matter how many buttons or widgets you use. See: https://developers.pinterest.com/pin_it/. I also added a line to set the data-pin-hover…

functions.php

// Add Pinterest Javascript to your website, see https://gist.github.com/Willem-Siebe/11351787.

function wsis_add_pinterest_script() {
    ?>
<script type="text/javascript">
(function(d){
    var f = d.getElementsByTagName('SCRIPT')[0], p = d.createElement('SCRIPT');
    p.type = 'text/javascript';
    p.async = true;
    p.setAttribute('data-pin-hover', true);
    p.src = '//assets.pinterest.com/js/pinit.js';
    f.parentNode.insertBefore(p, f);
}(document));
</script>
    <?php
}
add_action( 'wp_head', 'wsis_add_pinterest_script' );

On 12-04-2014 I discovered that when you are logged in with WordPress, there is a offset for the pin it button.