Use Flash 11's Sound::loadCompressedDataFromByteArray to load MP3 files

When available, use this method to load MP3s instead of our custom build-a-swf-in-memory method. For those with newer Flash versions this will fix https://github.com/LLK/scratch-flash/issues/206.
Is it always safe to use 44.1 here, even (for example) with a 22050Hz MP3?