Add Attribute and AttributeModifier API.

Thanks to __0x277F and Meeh on #spigot-dev for implementation advice.

By: md_5 <git@md-5.net>

paper-api/src/main/java/org/bukkit/attribute/Attribute.java

@@ -0,0 +1,48 @@
+package org.bukkit.attribute;
+
+/**
+ * Types of attributes which may be present on an {@link Attributable}.
+ */
+public enum Attribute {
+
+    /**
+     * Maximum health of an Entity.
+     */
+    GENERIC_MAX_HEALTH,
+    /**
+     * Range at which an Entity will follow others.
+     */
+    GENERIC_FOLLOW_RANGE,
+    /**
+     * Resistance of an Entity to knockback.
+     */
+    GENERIC_KNOCKBACK_RESISTANCE,
+    /**
+     * Movement speed of an Entity.
+     */
+    GENERIC_MOVEMENT_SPEED,
+    /**
+     * Attack damage of an Entity.
+     */
+    GENERIC_ATTACK_DAMAGE,
+    /**
+     * Attack speed of an Entity.
+     */
+    GENERIC_ATTACK_SPEED,
+    /**
+     * Armor bonus of an Entity.
+     */
+    GENERIC_ARMOR,
+    /**
+     * Luck bonus of an Entity.
+     */
+    GENERIC_LUCK,
+    /**
+     * Strength with which a horse will jump.
+     */
+    HORSE_JUMP_STRENGTH,
+    /**
+     * Chance of a zombie to spawn reinforcements.
+     */
+    ZOMBIE_SPAWN_REINFORCEMENTS;
+}