import integration spec for books
add Book model and migrate database
successfully pass the 'create' book spec
successfully pass the 'index' books spec
successfully pass the 'show' book spec
successfully pass the 'update' book spec
successfully pass the 'destroy' book spec
refactor all book routes to be resourceful
use url helper form book's resourceful routes
update to use url helper
use common form for both new and edit book pages
add scaffolding category
remove unused json responses from categories controller
apply one-to-many association to Category and Book models
add foreign key category_id for books
update book's spec to include category association
make all book specs pass again
add model validations for book
make books controller consist with categories controller
move flash message to application layout