Allow loading of plugin without AsyncWorldLoader

Just display a warning

src/net/hampoelz/NetherPortals/main/Main.java

@@ -21,7 +21,7 @@ public void onEnable()
         }
 		else
         {
-            throw new RuntimeException("[NetherPortals] Could not find AsyncWorldManager!! Plugin will likely not work without it! (If you want to try, remove the line that throws this exception)");
+            Bukkit.getLogger().warning("[NetherPortals] Could not find AsyncWorldManager!! Plugin will likely not work without it!");
         }
 
 		getCommands();