First 4 tests of products are workings

Author: fracz

src/main/java/pl/edu/agh/mwo/invoice/product/Product.java

@@ -16,18 +16,18 @@ protected Product(String name, BigDecimal price, BigDecimal tax) {
     }
 
     public String getName() {
-        return null;
+        return this.name;
     }
 
     public BigDecimal getPrice() {
-        return null;
+        return this.price;
     }
 
     public BigDecimal getTaxPercent() {
-        return null;
+        return this.taxPercent;
     }
 
     public BigDecimal getPriceWithTax() {
-        return null;
+        return this.price.multiply(this.taxPercent).add(this.price);
     }
 }