Add ItemStack array serialization methods

[kennytv]

Serializing multiple items to bytes isn't trivial, so often beginners fall into the trap of using output IO streams and Bukkit serialization instead, so these provide helpful extra methods to deal with item arrays.

Also kept the serialization methods exposed in API as a sort of textbook example/nudge in the right direction of what should be used instead of object streams

[kennytv]

While the api output still provides null items, I would not enforce air, otherwise you either produce inconsistent output from input, or break easy use from api input

[Kinqdos]

So whats the consensus now?