Add an immutable item api

[Techcable]

An immutable item API is necessary for us to replace bukkit's item api.
We should use inheritance, so items inherit from an abstract 'Item' class, avoiding the ItemMeta disaster.
With builders and withers, its an easy one-liner to construct them, even though they are immutable:

Item.builder(server, Materials.TACO)
        .tasty(true)
        .addEnchantment(EnchantmentTypes.SUPER_YUMMY, Integer.MAX_VALUE)
        .build()

• AbstractItem (actually called 'Item')
• nbt values/ItemMeta
  • Banner
  • Book
  • EnchantedBook
    • This is to represent the fact that the enchantments in enchanted books are not 'normal' enchants, they are 'stored' enchants
  • FireworkEffect
  • Firework
  • LeatherArmor
  • MapItem
  • PotionMeta
  • SkullMeta
  • SpawnerMeta
  • SpawnEggMeta
• data value abstractions
  • TODO

[phase]

Why is the server needed in the builder? I know plugin an just call Fountain.getServer, but why not do that in the constructor of the builder? What other server is possible?