Define mapped classes, World and Franchise.

1. Added 'director' to `Role`.
2. Added more detailed information to the docstring in `Work`.
3. Defined the mapped classes, `World` and `Franchise`.
4. Added the test on `World` and `Franchise`

Please do not merge now. It hasn't finished with the migration script for `World` and `Franchise` yet. I wish to merge this PR after merging #31.