Add OfflinePlayer, ItemEntity, Teleportable, Update Entity, Player, Damageable

* Updated JavaDocs for classes as well.
* ItemEntity:
 * Created class
* Entity:
 * Added methods getPosition(), getWorld(), teleport(vector3i, world), teleport(x, y, z, world)
* OfflinePlayer:
 * Created class
 * Added methods getName(), getUniqueID(), hasJoinedBefore(), isBanned(). isWhitelisted()
* Player:
 * Added methods hasJoinedBefore(), isBanned(), isWhitelisted()