Adds async="true" to your enqueued picturefil(.min).js

gistfile1.php

<?php
/**
 * Plugin Name: Picturefill Async
 * Description: Adds <code>async</code> to picturefill's enqueued js.
 * Version:     1.0
 * Required:    3.9
 * Author:      Donald Allen
 * Author URI:  http://donaldallen.com/
 * License:     GPL
 * License URI: http://www.gnu.org/copyleft/gpl.html
 */

if (!defined('ABSPATH')) {
    exit;
}

if (!function_exists('picturefill_async')) {
    function picturefill_async($url)
    {
        if (!strpos($url, 'picturefill')) {
            return $url;
        }

        return "$url' async='true";
    }

    add_filter('clean_url', 'picturefill_async', 11, 1);
}

Also, sample of using a device-pixel-ratio media query:

<picture>
    <source srcset="<?php echo $image_retina; ?>" media="only screen and (-webkit-min-device-pixel-ratio: 1.3), only screen and (min--moz-device-pixel-ratio: 1.3), only screen and (-o-min-device-pixel-ratio: 1.3 / 1), only screen and (min-resolution: 125dpi), only screen and (min-resolution: 1.3dppx)">
    <source srcset="<?php echo $image; ?>">
    <img srcset="<?php echo $image; ?>" alt="">
</picture>