Renamed the example scene.

1. Individual story files are now 'scenes', as that's a better name in
the runtime, and it's good to keep everything named consistently.
Wrote a simple Core class for the runtime.

1. Will wrap around an initialized instance of the runtime.
Updated Scene to depend on Core.

1. Also updated storymaster.h to include the other two headers - it will
be the 'main' header for client software to include.