Added Difficulty API.

By: EvilSeph <evilseph@gmail.com>

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

@@ -0,0 +1,61 @@
+package org.bukkit;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Represents the various difficulty levels that are available.
+ */
+public enum Difficulty {
+    /**
+     * Players regain health over time, hostile mobs don't spawn, the hunger bar does not deplete.
+     */
+    PEACEFUL(0),
+
+    /**
+     * Hostile mobs spawn, enemies deal less damage than on normal difficulty, the hunger bar does deplete and starving deals up to 5 hearts of damage. (Default value)
+     */
+    EASY(1),
+
+    /**
+     * Hostile mobs spawn, enemies deal normal amounts of damage, the hunger bar does deplete and starving deals up to 9.5 hearts of damage.
+     */
+    NORMAL(2),
+
+    /**
+     * Hostile mobs spawn, enemies deal greater damage than on normal difficulty, the hunger bar does deplete and starving can kill players.
+     */
+    HARD(3);
+
+    private final int value;
+    private final static Map<Integer, Difficulty> difficulties = new HashMap<Integer, Difficulty>();
+
+    private Difficulty(final int value) {
+        this.value = value;
+    }
+
+    /**
+     * Gets the difficulty value associated with this Difficulty.
+     *
+     * @return An integer value of this difficulty
+     */
+    public int getValue() {
+        return value;
+    }
+
+    /**
+     * Gets the Difficulty represented by the specified value
+     *
+     * @param value Value to check
+     * @return Associative {@link Difficulty} with the given value, or null if it doesn't exist
+     */
+    public static Difficulty getByValue(final int value) {
+        return difficulties.get(value);
+    }
+
+    static {
+        for (Difficulty diff : Difficulty.values()) {
+            difficulties.put(diff.getValue(), diff);
+        }
+    }
+}