Add OfflinePlayer, Update Player, Entity, Item

* Updated JavaDocs for classes as well.
* Entity:
 * Added methods getPosition(), getWorld(), teleport(vector3i), teleport(x, y, z)
* OfflinePlayer:
 * Created class
 * Added methods getName(), getUniqueID(), hasJoinedBefore(), isBanned(). isWhitelisted()
* Player:
 * Added methods hasJoinedBefore(), isBanned(), isWhitelisted()
* Item:
 * Extends Entity interface
It was suggested that teleport should return something, see here: https://github.com/SpongePowered/SpongeAPI/commit/d53ed206f9bffba1fabd7cf440ad49b2099f3734#commitcomment-7715476