It seemed like a crazy idea. When Radiohead said it would release its new album, In Rainbows, as a pay-what-you-will digital download, you’d have thought the band had gone communist. After all, Thom Yorke and company are one of the world’s most successful groups — a critical darling as well as a fan favorite for nearly 15 years. They hadn’t put out a new album in more than four years, and the mark