Added GameMode methods to Server and HumanEntity

By: Dinnerbone <dinnerbone@dinnerbone.com>

paper-api/src/main/java/org/bukkit/GameMode.java

@@ -0,0 +1,18 @@
+package org.bukkit;
+
+import org.bukkit.entity.HumanEntity;
+
+/**
+ * Represents the various type of game modes that {@link HumanEntity}s may have
+ */
+public enum GameMode {
+    /**
+     * Creative mode may fly, build instantly, become invulnerable and create free items
+     */
+    CREATIVE,
+    
+    /**
+     * Survival mode is the "normal" gameplay type, with no special features.
+     */
+    SURVIVAL;
+}