add mp3 to audio_pygame.py

mp3 is supported on android (tested) 
http://developer.android.com/guide/appendix/media-formats.html

iOS should be supported too (haven't tested) 
http://developer.apple.com/library/ios/#documentation/AudioVideo/Conceptual/MultimediaPG/UsingAudio/UsingAudio.html

I don't know how to "a,b,c on android; x,y,z on iOS" so I can't add the rest of the formats.