Automatically embed Dribbble shots into WordPress content

dribbble-shot-embedder.php

<?php
/*
Plugin Name: Dribbble Shot Embed
Description: Automatically embed a shot into your WordPress site just by dropping the URL in place.
Version: 1.0.0.RC.1
Author: Nick Ohrn of Plugin-Developer.com
Author URI: http://plugin-developer.com/
*/

function dribbble_shot_embed_callback($matches, $attr, $url, $rawattr) {
	$output = false;

	if(($id = isset($matches[1]) ? $matches[1] : false)) {
		$key = sprintf('dribbble_shot_%d', $id);
		$shot = get_transient($key);

		if(false === $shot) {
			$response = wp_remote_get(sprintf('http://api.dribbble.com/shots/%d', $id));
			if(!is_wp_error($response)) {
				$shot = json_decode(wp_remote_retrieve_body($response), true);

				if($shot) {
					set_transient($key, $shot, DAY_IN_SECONDS);
				}
			}
		}

		if($shot) {
			$title = esc_attr($shot['title']);
			$url = esc_attr(esc_url($shot['url']));

			$image_height = absint($shot['height']);
			$image_src = esc_attr(esc_url($shot['image_url']));
			$image_width = absint($shot['width']);

			$output = sprintf('<a href="%1$s" target="_blank" title="%2$s"><img alt="%2$s" src="%3$s" height="%4$d" width="%5$d" /></a>', $url, $title, $image_src, $image_height, $image_width);
		}
	}

	return $output;
}

function dribbble_shot_embed_wp_embed_register_handler() {
	wp_embed_register_handler('dribbble-shot-embed', '#^http://dribbble.com/shots/(\d+)#i', 'dribbble_shot_embed_callback');
}
add_action('init', 'dribbble_shot_embed_wp_embed_register_handler');

Doesn't work.