Add commit method

src/main/java/Purchase.java

@@ -0,0 +1,22 @@
+import java.math.BigDecimal;
+
+public class Purchase extends Transaction {
+    public Purchase(Share share, int week) {
+        super(share, week, new PurchaseCalculator(share));  
+    }
+
+    @Override
+    public void commit(Player player) {
+        BigDecimal price = this.getShare().getPurchasePrice();
+
+        if (isCommitted() || (player.getMoney().compareTo(price) < 0)) {
+            return;
+        }
+
+        player.withdrawMoney(price);
+        player.getPortfolio().addShare(getShare());
+        player.getTransactionArchive().add(this);
+
+        this.committed = true;
+    }
+}