Implement Router decorator for polymorphic resource routing.

- Refactor Appleseed::ResourceNotFoundError to
Appleseed::ResourcesNotFoundError: pluralize name, enforce support for
multiple missing resources.

Current spec status: 763 examples, 0 failures in 7.37 seconds.
Implement DirectoryRouter.

Current spec status: 773 examples, 0 failures in 7.46 seconds.
Refactor DirectoryLookup concern to use router objects.

Current spec status: 782 examples, 0 failures in 7.21 seconds.
Delegate incomplete routing to child router.

Current spec status: 786 examples, 0 failures in 7.49 seconds.
Merge pull request #13 from sleepingkingstudios/feature-routing-fallback

Implement Router decorator for polymorphic resource routing.