Add Entity/Player interfaces

First steps in entity/player interfaces - returning the UUID for all, and last
known name/display name for the Player.

`Entity` -> `LivingEntity` -> `HumanEntity` -> `Player`
World/Chunk load/unload events

Adds abstract classes for `World`/`Chunk` load and unload events.
Merge pull request #5 from prism/interface-entity

Add entity/Player interfaces
Merge pull request #20 from prism/events-world

World/Chunk load/unload events