Add author and id methods to Book class
Add status to Book class of library.rb. Add arguments of library_spec.rb
Add check_in method to Book class. Modify 'can't be checked out' test
Add name method to Borrower class
Add attr_reader name, books
Modify register_new_book method on Library class to include id & books array
Update check_out_book of Library class
Modify check_in_book and check_out_book methods in Library class