Added BukkitMultiverseWorld implementation.

bukkit/src/main/java/com/onarandombox/multiverse/core/BukkitWorld.java

@@ -0,0 +1,39 @@
+package com.onarandombox.multiverse.core;
+
+import com.onarandombox.multiverse.core.api.BukkitMultiverseWorld;
+import com.onarandombox.multiverse.core.api.WorldProperties;
+import org.bukkit.Bukkit;
+import org.bukkit.World;
+
+import java.util.UUID;
+
+class BukkitWorld extends AbstractMultiverseWorld implements BukkitMultiverseWorld {
+
+    private final String name;
+    private final UUID worldUID;
+
+    BukkitWorld(final World world, final WorldProperties worldProperties) {
+        super(worldProperties);
+        this.name = world.getName();
+        this.worldUID = world.getUID();
+    }
+
+    @Override
+    public String getName() {
+        return this.name;
+    }
+
+    @Override
+    public UUID getWorldUID() {
+        return this.worldUID;
+    }
+
+    @Override
+    public World getBukkitWorld() {
+        final World world = Bukkit.getWorld(worldUID);
+        if (world == null) {
+            throw new NullPointerException("Multiverse lost track of Bukkit world '" + this.name + "'");
+        }
+        return world;
+    }
+}