MP3 support added

I've added MP3 support to the audio system (more specifically the SoundResourceHandle) using the MAD library. I'd like to implement streaming MP3's from memory, this should be possible using MAD although for now I probably won't bother. The only problem is that a 4mb MP3 could expand to a 50mb sound file in memory, although with the obscene amounts of memory people have these days it shouldn't be a problem (it will become a problem if I port the game to any other systems).