#1 - 404 on scripts and stylesheets

Admittedly, my WordPress installation is idiosyncratic, and the issue would only affect a fraction of users, but I believe the code you used to support ancient versions of WP can be safely retired at this point and that the replacement code utilizing `plugin_dir_url()` is simpler, more reliable, and more compliant with best practices.

The plugin was broken for me, with the following client-side errors, before the patch.

    GET http://localhost/***.amazonaws.com/~ubuntu/content/plugins/wordpress-console/console.css?ver=3.6-alpha-23257 404 (Not Found) admin.php:29
    GET http://localhost/***.amazonaws.com/~ubuntu/content/plugins/wordpress-console/console.js?ver=3.6-alpha-23257 404 (Not Found) admin.php:30
    GET http://localhost/***.amazonaws.com/~ubuntu/content/plugins/wordpress-console/sha1.js?ver=3.6-alpha-23257 404 (Not Found) admin.php:30