Implement tab completion for all commands. (#1282)

List of supported commands:
```
/afk
/balance
/balancetop
/ban
/banip
/bigtree
/book
/broadcastworld
/burn
/clearinventory
/condense
/delhome
/deljail
/delwarp
/eco
/enchant
/enderchest
/essentials
/exp
/ext
/feed
/fireball
/firework
/gamemode
/getpos
/give
/hat
/heal
/help
/helpop
/home
/ignore
/invsee
/item
/itemdb
/jump
/kick
/kill
/kit
/lightning
/list
/mail
/me
/msg
/mute
/near
/nick
/nuke
/pay
/potion
/powertool
/ptime
/pweather
/recipe
/remove
/repair
/sell
/showkit
/skull
/speed
/tempban
/thunder
/time
/togglejail
/tp
/tpa
/tpaall
/tpahere
/tpall
/tphere
/tpo
/tpohere
/tppos
/tree
/warp
/weather
/world
/worth```

Essentials/src/com/earth2me/essentials/commands/Commandbook.java

@@ -2,11 +2,15 @@ package com.earth2me.essentials.commands;
 
 import com.earth2me.essentials.User;
 import com.earth2me.essentials.craftbukkit.InventoryWorkaround;
+import com.google.common.collect.Lists;
 import org.bukkit.Material;
 import org.bukkit.Server;
 import org.bukkit.inventory.ItemStack;
 import org.bukkit.inventory.meta.BookMeta;
 
+import java.util.Collections;
+import java.util.List;
+
 import static com.earth2me.essentials.I18n.tl;
 
 
@@ -67,4 +71,25 @@ public class Commandbook extends EssentialsCommand {
         String author = bmeta.getAuthor();
         return author != null && author.equalsIgnoreCase(player);
     }
+
+    @Override
+    protected List<String> getTabCompleteOptions(Server server, User user, String commandLabel, String[] args) {
+        // Right now, we aren't testing what's held in the player's hand - we could, but it's not necessarily worth it
+        if (args.length == 1) {
+            List<String> options = Lists.newArrayList("sign", "unsign");  // sign and unsign aren't real, but work
+            if (user.isAuthorized("essentials.book.author")) {
+                options.add("author");
+            }
+            if (user.isAuthorized("essentials.book.title")) {
+                options.add("title");
+            }
+            return options;
+        } else if (args.length == 2 && args[0].equalsIgnoreCase("author") && user.isAuthorized("essentials.book.author")) {
+            List<String> options = getPlayers(server, user);
+            options.add("Herobrine"); // #EasterEgg
+            return options;
+        } else {
+            return Collections.emptyList();
+        }
+    }
 }