Battle music volume is set based on the music volume slider in the
Minecraft settings. The default Midi device provided in Java does not
appear to support changing the volume of playing midi file after some
testing. For now, only ".wav" files obey the Music volume slider
settings.
BattleGui now displays health of all combatants.
Players can leave battle by entering creative mode (set by server).
Added battle size limit (default 8) can be set in config.