added basic math

src/main/java/rocks/zipcode/calcskin/CalcEngine.java

     CalcEngine() {
     }
 
-    public double add(double v, double v1) {
-        return Double.NaN;
-    }
+    public double add(double v, double v1) { return (v+v1);}
 
-    public double subtract(double v, double v1) {
-        return Double.NaN;
-    }
+    public double subtract(double v, double v1) { return (v-v1);}
 
-    public double multiply(double v, double v1) {
-        return Double.NaN;
-    }
+    public double multiply(double v, double v1) { return (v*v1);}
 
-    public double divide(double v, double v1) {
-        return Double.NaN;
-    }
+    public double divide(double v, double v1) { return (v/v1);}
 }