Create WorldSaveEvent listener that stores challenges, when event occur.

Partial fix of issue #21.

src/main/java/bentobox/addon/challenges/ChallengesAddon.java

@@ -5,6 +5,7 @@
 import bentobox.addon.challenges.commands.ChallengesCommand;
 import bentobox.addon.challenges.commands.admin.Challenges;
 import bentobox.addon.challenges.listeners.ResetListener;
+import bentobox.addon.challenges.listeners.SaveListener;
 import world.bentobox.bentobox.api.addons.Addon;
 import world.bentobox.bentobox.api.commands.CompositeCommand;
 
@@ -73,6 +74,8 @@ public void onEnable() {
         }
         // Register the reset listener
         this.registerListener(new ResetListener(this));
+        // Register the autosave listener.
+        this.registerListener(new SaveListener(this));
         // Done
     }
 

src/main/java/bentobox/addon/challenges/listeners/SaveListener.java

@@ -0,0 +1,36 @@
+package bentobox.addon.challenges.listeners;
+
+
+import org.bukkit.event.EventHandler;
+import org.bukkit.event.EventPriority;
+import org.bukkit.event.Listener;
+import org.bukkit.event.world.WorldSaveEvent;
+
+import bentobox.addon.challenges.ChallengesAddon;
+
+
+/**
+ * This is Simple World Save event listener. On each world save, this method
+ * asks challenge manager to save its data.
+ */
+public class SaveListener implements Listener
+{
+	public SaveListener(ChallengesAddon addon) {
+		this.addon = addon;
+	}
+
+
+	@EventHandler(priority = EventPriority.LOW, ignoreCancelled = true)
+	public void onWorldSave(WorldSaveEvent e)
+	{
+		this.addon.getChallengesManager().save(e.isAsynchronous());
+	}
+
+
+// ---------------------------------------------------------------------
+// Section: Variables
+// ---------------------------------------------------------------------
+
+
+	private ChallengesAddon addon;
+}