Allow use of "config name" in default-class.

This is a very simple change that could potentially save a lot of
headaches when people look elsewhere in their config-file and see a
class name like "Big Gary" and wonder why putting that value in the
`default-class` property won't work.

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

@@ -191,7 +191,8 @@ public class ArenaImpl implements Arena
 
         String defaultClassName = settings.getString("default-class", null);
         if (defaultClassName != null) {
-            this.defaultClass = classes.get(defaultClassName);
+            String slug = Slugs.create(defaultClassName);
+            this.defaultClass = classes.get(slug);
         }
 
         // Blocks and pets