Parallax Camera

[danhealy]

It should be fairly easy to modify Zif::Camera to implement a parallax effect (e.g. flappy dragon)

This might be a good first project for a new contributor:

1. Create a ParallaxLayer class based on SimpleLayer ActiveLayer
2. Create an example scene similar to the World scene which initializes some ParallaxLayers
3. Hook the new example scene into the sequence somewhere by registering the scene in main.rb and then modifying the @next_scene attributes on the existing example scenes.