Add support for Apple's Audiobook formats m4a and m4b

The description of this software says "It is primarily intended for accelerating audio books and podcasts...". Therefore it should also support the audio book format, .m4b, used by Apple's iPod/iPhone/iPad. The .m4b format is, as far as I can see identical to Apple's .m4a format, except of the naming difference. These devices can play .mp3, but that format is highly inconvenient for audio books, as you won't get a bookmark to let you resume playback where you stopped last time you where listening. Uploading mp3 files on your iPod will put it under music. Uploading .m4b files will put them under audio books, and provide bookmarking to them.