Remove audio_file_transfer() and use drupal_goto() to redirect to download

I wrote the code to use audio_file_transfer() back when I didn't know any better. Now I know what a bad idea it is in terms of killing file resuming, performance, etc. But I like the idea of keeping the play/download statistics (as well as not breaking any existing URLs) so I think we can just have audio_play() and audio_download() call drupal_goto(file_create_url($node->audio_file['file_path'])); and get the same effect. We'll also need to implement hook_file_download() so that private file transfers work.