Use class "config name" on leaderboard signs.

Same as with arena names, we can just use the class "config name" on
leaderboards for a prettier experience.

src/main/java/com/garbagemule/MobArena/ArenaPlayerStatistics.java

@@ -17,7 +17,7 @@ public class ArenaPlayerStatistics
     public ArenaPlayerStatistics(ArenaPlayer player) {
         this.player = player;
         this.playerName = player.getPlayer().getName();
-        this.className = player.getArenaClass().getLowercaseName();
+        this.className = player.getArenaClass().getConfigName();
         reset();
     }