Add more item-related functions

Add forgotten license
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.
Adding BlockEvent abstract class

Adding a base `BlockEvent` interface.
Add Contribution Guidlines link to README
`HumanEntity` properly extendeds `LivingEntity`

We somehow missed this in the last PR.
`getChunk` clarifications, `loadChunk` method

Based on @mikeprimm feedback on c34d11d0, renaming chunk coordinates in method sig to `cx`, etc.

Clarifying that `getChunk` returns null if it's not loaded.

Adding a `loadChunk` method, with optional bool to disable generating.
Add Voxel class

Got to merge that math branch…
Add Vector3i getters for Chunk and World