Added I18NBundleLoader.

It works, but I think we have a general problem with bundle loaders.
I mean, the 2 bundles below have the same base name but different locale
````java
manager.load("data/i18n/message1", I18NBundle.class, new I18NBundleParameter(Locale.ITALIAN));
manager.load("data/i18n/message1", I18NBundle.class, new I18NBundleParameter(Locale.ENGLISH));
````
The second bundle won't be loaded.
How to fix this?
If the fix is too expensive we might just put an alert on the asset loader wiki page since this is not a common case.