Add overlay div for neatly displaying the pieces.

Go/View.elm

@@ -15,7 +15,7 @@ renderIndex (index, color) =
         div [ classList [ ("board-cell", True), extraClass ]
             , onClick (Place index)
             ]
-            [ 
+            [ div [ class "overlay" ] []
             ]
 
 renderBoard : Int -> Board -> Html Msg