Resolve "Init String calculator project"

Closes #11 (closed)

stringCalculator/src/main/java/fr/ippon/stringcalculator/Calculator.java

+package fr.ippon.stringcalculator;
+
+import java.math.BigDecimal;
+import java.util.function.BinaryOperator;
+
+public class Calculator {
+
+  public static String sum(String first, String second) {
+    return operate(first, second, BigDecimal::add);
+  }
+
+  public static String substract(String first,
+      String second) {
+    return operate(first, second, BigDecimal::subtract);
+  }
+
+  private static String operate(String first,
+      String second, BinaryOperator<BigDecimal> operation) {
+    notNull(first);
+    notNull(second);
+
+    return operation
+        .apply(toBigDecimal(first), toBigDecimal(second))
+        .toPlainString();
+  }
+
+  private static BigDecimal toBigDecimal(String value) {
+    if (value.isBlank()) {
+      return BigDecimal.ZERO;
+    }
+
+    return new BigDecimal(value.replace(",", "."));
+  }
+
+  private static void notNull(String first) {
+    if (first == null) {
+      throw new IllegalArgumentException();
+    }
+  }
+
+}