Update with new sprites and features

This update attempts to demonstrate "Experiential Instruction" in level design by introducing a cloud into the environment.

When the plane is in collision with the cloud, the gravity scale is modified and the plane will fall more slowly. With practice it is possible to float on the cloud as it moves back and forth across the screen. When the plane and cloud are no longer in collision, the gravity will return to normal and the plane will once again fall more quickly.