"Backport" modern Inventory API

[zavdav]

Attempt to implement the Inventory API found in newer versions of Bukkit

[RhysB]

Thsi is beautiful

[sshshark]

Upon trying to connect to the server this error is printed when attempting to log in.

[WARNING] Failed to handle packet: java.lang.NullPointerException: key
java.lang.NullPointerException: key
        at com.google.common.collect.CustomConcurrentHashMap$Impl.get(CustomConcurrentHashMap.java:1328)
        at org.bukkit.craftbukkit.entity.CraftEntity.getPlayer(CraftEntity.java:270)
        at org.bukkit.craftbukkit.entity.CraftEntity.getEntity(CraftEntity.java:36)
        at net.minecraft.server.Entity.getBukkitEntity(Entity.java:1174)
        at net.minecraft.server.ContainerPlayer.getBukkitView(ContainerPlayer.java:126)
        at net.minecraft.server.ContainerPlayer.a(ContainerPlayer.java:58)
        at net.minecraft.server.ContainerPlayer.<init>(ContainerPlayer.java:53)
        at net.minecraft.server.EntityHuman.<init>(EntityHuman.java:60)
        at net.minecraft.server.EntityPlayer.<init>(EntityPlayer.java:37)
        at net.minecraft.server.ServerConfigurationManager.a(ServerConfigurationManager.java:195)
        at net.minecraft.server.NetLoginHandler.b(NetLoginHandler.java:159)
        at net.minecraft.server.NetLoginHandler.a(NetLoginHandler.java:52)
        at net.minecraft.server.NetworkListenThread.a(NetworkListenThread.java:49)
        at net.minecraft.server.MinecraftServer.h(MinecraftServer.java:614)
        at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:492)
        at net.minecraft.server.ThreadServerApplication.run(ThreadServerApplication.java:13)

[thecow275]

requesting re review from sshshark