[#3] Refactor, pull reduced() as interface method and impl it in Money

src/main/java/org/thoughtworks/linesh/multicurrencymoney/Bank.java

@@ -3,7 +3,7 @@
 public class Bank {
     public Money reduced(Expression money, String toCurrency) {
         // TODO: [Linesh][1/9/17] Is type cast a bad smell? 
-        if (money instanceof Money) return (Money) money;
+        if (money instanceof Money) return money.reduced(toCurrency);
         Sum sum = (Sum) money;
         return sum.reduced(toCurrency);
     }

src/main/java/org/thoughtworks/linesh/multicurrencymoney/Expression.java

@@ -2,4 +2,6 @@
 
 public interface Expression {
     Expression plus(Money money);
+
+    Money reduced(String currency);
 }

src/main/java/org/thoughtworks/linesh/multicurrencymoney/Money.java

@@ -44,4 +44,9 @@ public Money times(int multiplier) {
     public Expression plus(Money addend) {
         return new Sum(this, addend);
     }
+
+    @Override
+    public Money reduced(String currency) {
+        return this;
+    }
 }