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.
Merge branch 'master' into feature-blogs
Implement Admin::Features::BlogsController #index, #new, #create.

Current spec status: 851 examples, 0 failures in 9.6 seconds.
Implement Blog #edit, #update, #destroy.

Current spec status: 862 examples, 0 failures in 8.84 seconds.
Implement Blog#show, Blog views.

Current spec status: 865 examples, 0 failures in 8.84 seconds.