Added register_new_book method to Library.
Added method to allow a book to be checked out by its ID
Added method to know who borrowed a book.
Modified Library instance method 'check_out_book' to prevent checking out one book twice.
Added check_in_book method to Library class.
Edited Library.check_out_book method to prevent a borrower from checking out more than two books; added a helper method and books instance variable to Borrower class.
Created available_books and borrowed_books instance varaibles for Library class.