Exploration: a self-contained default scene

[mikerreed]

Can we offer an API that is optimized for the "just play the right thing" client?

1. Get the file bytes
2. Create the "default" scene
3. go

This is possible because of two things:

1. This particular Scene subclass "owns" its underlying file and artboardinstance
2. Scenes can do everything a basic client needs: bounds, draw, advance, inputs, pointers