Add NormalItem with updateQuality logic

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

@@ -0,0 +1,15 @@
+package com.gildedrose.items;
+
+import com.gildedrose.Item;
+
+public class NormalItem extends GildedItem {
+    public NormalItem(Item item) { super(item); }
+
+    @Override
+    public void updateQuality() {
+        decreaseQuality(1);
+        decreaseSellIn();
+        if (item.sellIn < 0)
+            decreaseQuality(1);
+    }
+}