Separate sellIn behavior in separate class

We extract this behavior in a separate interface so we can implement
different implementations of it and test them separately.
Default behavior is to decrease the sellIn date every iteration by 1

Java/src/main/java/com/gildedrose/behavior/sellin/DefaultSellInBehavior.java

@@ -0,0 +1,15 @@
+package com.gildedrose.behavior.sellin;
+
+import com.gildedrose.Item;
+
+public class DefaultSellInBehavior implements SellInBehavior {
+
+    @Override
+    public void processSellInUpdate(Item item) {
+        decreaseSellIn(item);
+    }
+
+    private void decreaseSellIn(Item item) {
+        item.sellIn = item.sellIn - 1;
+    }
+}

Java/src/main/java/com/gildedrose/behavior/sellin/SellInBehavior.java

@@ -0,0 +1,8 @@
+package com.gildedrose.behavior.sellin;
+
+import com.gildedrose.Item;
+
+public interface SellInBehavior {
+
+    void processSellInUpdate(Item item);
+}