Add: `LayerTouchable` module for `Joybox::Core::Layer{,Color}`
Rename: `LayerTouchable` module to `TouchEvent`
Add event module separated from `layer.rb`. Add LayerColor.
Add specs for events.
Merge pull request #57 from meganemura/fix-joybox-ios-core-layer

Add:
=> Separate some methods about touch from `motion/joybox-ios/core/layer.rb` for mix-in to other Layer class