fix Magnatune plugin

Magnatune changed it's layout and dropped all meta data. Therefore this plugin can no longer extract the album description from the open graph meta data. I could extract it from the html body. Should I?

Anyway, the new html is horribly broken (not doctype, no </head>, lot's of deprecated old tags/attributes, script elements between </body> and </html> etc.). It's a wonder the site works as good as it does.