add, subtract, multiply and divide functions added

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

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

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

         final TextField screen  = createScreen();
         final TilePane  buttons = createButtons();
 
-        stage.setTitle("Calc");
+        stage.setTitle("Michelle's Calculator");
         stage.initStyle(StageStyle.UTILITY);
         stage.setResizable(false);
         stage.setScene(new Scene(createLayout(screen, buttons)));