Add a message prefix to avoid plugin confusion

Due to BattleArena's selfish and obtrusive sign handling, it is not
possible to place signs where the first line of the sign contains
a name of a BattleArena class without BattleArena interfering. With
this and the fact that both MobArena and BattleArena have a default
class called Archer, MobArena users very frequently complain that
either "only the Archer sign activates" (because BattleArena steals
the sign) or "the only class that doesn't work is Archer" (because
BattleArena manipulates the contents of the sign).

This commit adds a message prefix to the MessageUtil class so all
messages from BattleArena are prefixed with [BattleArena]. Changing
this prefix is very easy and could easily be made customizable. It
does, however, prevent any confusion as to which plugin the message
comes from.