Add borrower_books array to Borrower, Update checkout method for
Library to add books to borrower_books
Add borrowed_books_list to return list of Borrower's books.
Add available_books method to show list of books that are available.

Also updated formatting for displaying the books in all list.
Add check_in method to check in books, make available for check out and
remove from Borrower's borrwed_books array.

Add borrowed_books_count method to return number of books borrowed.

Update check_out method to included borrowed_books_count

Update main.rb to test check_in method
Add year_published and edition to parameters for Book.new

Add review method to allow users to leave reviews.
Add return_date to book

Update check_out method to set return_date to a week from time now.

Add none_overdue? method to borrower to return true if no books are
over due and false if there are books over due.
Update check_in to update return_date to Time.now