[Bleeding] Implementation of inventory framework. Addresses BUKKIT-856

See the corresponding Bukkit commit for details.

Implementation details:
- Any packets that include an itemstack will send air stacks as null; maybe this will even eliminate the client crash that occurs if the client receives an air stack
- Better handling of null itemstacks in general (ie less converting them to air stacks)
- Inventory.setContents() can now take an array smaller than the inventory without error
- Player.updateInventory() should now correctly update the result slot in a crafting inventory

Some small credit goes to Afforess (initial implementation of openInventory() methods) and Drakia (initial implementation of InventoryOpenEvent and InventoryCloseEvent).

src/main/java/net/minecraft/server/Packet.java

@@ -157,7 +157,7 @@ public abstract class Packet {
     }
 
     protected void a(ItemStack itemstack, DataOutputStream dataoutputstream) throws IOException { // CraftBukkit
-        if (itemstack == null) {
+        if (itemstack == null || itemstack.id <= 0) {
             dataoutputstream.writeShort(-1);
         } else {
             dataoutputstream.writeShort(itemstack.id);