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`
Update README PR guidelines

Based on feedback so far
World/Chunk load/unload events

Adds abstract classes for `World`/`Chunk` load and unload events.
Adding BlockEvent abstract class

Adding a base `BlockEvent` interface.
Merge pull request #5 from prism/interface-entity

Add entity/Player interfaces
Add Contribution Guidlines link to README
Merge pull request #20 from prism/events-world

World/Chunk load/unload events
Merge pull request #34 from prism/events-block

Adding BlockEvent abstract class
Merge pull request #39 from simon816/readme-guideline-link

Add Contribution Guidlines link to README
Merge pull request #2 from SpongePowered/master

Update from original