Fix: Modify `Joybox::Core::LayerColor.new` to return LayerColor instance...

Hi.

`Joybox::Core::LayerColor.new` is expected to return `Joybox::Core::LayerColor` instance.
However, it returns CCLayerColor, and its instance is not callable `on_enter` method of course.
This c679069 commit fixes this.

and..
  * modify defined `Joybox::Core::LayerColor.scene` to accept arguments for color.
  * add spec

thanks.