basic implementation

Introduce the `Calculator` class with single member-function which
performs an addition.

Signed-off-by: Sergii Piatakov <sergii.piatakov@globallogic.com>

calculator.cpp

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+#include "calculator.h"
+
+int Calculator::Add (int a, int b)
+{
+    return a + b;
+}

calculator.h

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+class Calculator
+{
+    public:
+        int Add (int, int);
+};