Improve support for the toString() method in wrapped objects.
Use the object wrappers when printing custom Minecraft objects.
Only add the WorldType converter if it exists in that Minecraft version
Added support for modifying attributes in UPDATE_ATTRIBUTES.

This contains a fully-fledged API for reading and modifying Attribute-
Snapshot and AttributeModifier. Keep in mind that these objects are 
immutable, so modification must be made through object builders.

The packets are also shared, so packet cloning might be necessary if 
attributes should differ per player.