WPPlugin::url_to_plugin_directory doesn't work correctly

There is a WordPress library function called [plugins_url](http://codex.wordpress.org/Function_Reference/plugins_url) which works fine. This patch uses the library function.

I ran into this because I'm running the wordpress code from a separate subfolder, as outlined in this blog post: http://davidwinter.me/articles/2012/04/09/install-and-manage-wordpress-with-git/