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

  * defined `Joybox::Core::LayerColor.scene` should accept arguments for color.
  * add spec
Merge pull request #53 from meganemura/fix-layercolor-new-and-scene

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