Times adjustments on Scene
Move timing to constants & instance properties
Merge pull request #4 from rafaelks/times-adjustments

Improvement: use timing from constants & instance properties
Create a new class to Background SpriteNode
Start using new class on PlayingScene
Merge pull request #5 from rafaelks/separate-nodes-classes

Improvement: separate BackgroundSpriteNode in a separated class
Create a AirPlaneObstacle class & images
Add Obstacles to BackgroundNodes
Adjustments on Obstacles
Resolve conflicts