Add base GildedItem abstract class for item hierarchy

Java/src/main/java/com/gildedrose/items/GildedItem.java

@@ -0,0 +1,26 @@
+package com.gildedrose.items;
+
+import com.gildedrose.Item;
+
+public abstract class GildedItem {
+
+    protected Item item;
+
+    public GildedItem(Item item) {
+        this.item = item;
+    }
+
+    public abstract void updateQuality();
+
+    protected void decreaseQuality(int amount) {
+        item.quality = Math.max(0, item.quality - amount);
+    }
+
+    protected void increaseQuality(int amount) {
+        item.quality = Math.min(50, item.quality + amount);
+    }
+
+    protected void decreaseSellIn() {
+        item.sellIn--;
+    }
+}