Move version command update checking to the implementation

This makes it easier for downstream projects (forks) to replace the
version fetching system with their own. It is as simple as implementing
an interface and overriding the default implementation of
org.bukkit.UnsafeValues#getVersionFetcher()

It also makes it easier for us to organize things like the version
history feature.

Lastly I have updated the paper implementation to check against the site
API rather than against jenkins.

Spigot-Server-Patches/0137-Optimize-ItemStack.isEmpty.patch

@@ -0,0 +1,23 @@
+From 3820ee666676a9a9a03b771e6235c28375663d42 Mon Sep 17 00:00:00 2001
+From: Aikar <aikar@aikar.co>
+Date: Wed, 21 Dec 2016 03:48:29 -0500
+Subject: [PATCH] Optimize ItemStack.isEmpty()
+
+Remove hashMap lookup every check, simplify code to remove ternary
+
+diff --git a/src/main/java/net/minecraft/server/ItemStack.java b/src/main/java/net/minecraft/server/ItemStack.java
+index b643a4123..b7d327211 100644
+--- a/src/main/java/net/minecraft/server/ItemStack.java
++++ b/src/main/java/net/minecraft/server/ItemStack.java
+@@ -145,7 +145,7 @@ public final class ItemStack {
+     }
+ 
+     public boolean isEmpty() {
+-        return this == ItemStack.a ? true : (this.getItem() != null && this.getItem() != Items.AIR ? this.count <= 0 : true);
++        return this == ItemStack.a || this.item == null || this.item == Items.AIR || this.count <= 0; // Paper
+     }
+ 
+     public ItemStack cloneAndSubtract(int i) {
+-- 
+2.21.0
+