Add getAPIVersion and getImplementationVersion

Signed-off-by: Steven Downer <grinch@outlook.com>
Merge pull request #12 from Grinch/master

Add getAPIVersion and getImplementationVersion
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`