Add OfflinePlayer, Update Player, Entity, Item

* Updated JavaDocs for classes as well.
* Entity:
* * Added methods getVoxel(), getWorld(), teleport()
* OfflinePlayer:
* * Created class
* * Added methods getName(), getUniqueID(), hasJoinedBefore(), isBanned(). isWhitelisted()
* Player:
* * Added methods hasJoinedBefore(), isBanned(), isWhitelisted()
* Item:
* * Extends Entity interface