[Bleeding] Added Vanish API for hiding players from each other.

Adds Player.hidePlayer, Player.showPlayer, and Player.canSee for managing
what players are hidden from a player. When someone is hidden from a player
the player cannot see them in the user list or /list and they cannot /tell
them so they appear to be completely gone from the server.

src/test/java/org/bukkit/plugin/messaging/TestPlayer.java

@@ -609,4 +609,16 @@ public class TestPlayer implements Player {
     public void setBedSpawnLocation(Location location) {
         throw new UnsupportedOperationException("Not supported yet.");
     }
+
+    public void hidePlayer(Player player) {
+        throw new UnsupportedOperationException("Not supported yet.");
+    }
+
+    public void showPlayer(Player player) {
+        throw new UnsupportedOperationException("Not supported yet.");
+    }
+
+    public boolean canSee(Player player) {
+        throw new UnsupportedOperationException("Not supported yet.");
+    }
 }