Add Entity

Added entity class with most field getters and setters.
Will support mods as long as the entity type can be modified by the server at any time.
@kitskub One of my own coding habits.
When i am coding something that requires the entity to be alive.
I always asume there is entity.isAlive
But when im checking if he is dead i am asuming there is isDead.
Besides, i couldn't decide which one is better :3