Song IO Reference

BtSongIO — base class for song input and output
BtSongIONative — class for song input and output in builtin native format
BtSongIONativeBZT — class for song input and output in builtin native format
BtSongIONativeXML — class for song input and output in builtin native format
BtSongIOBuzz — class for song input in buzz bmx and bmw format