Entities:
Author
Name - required, String, not empty
Biography - optional
Book
Title - required, String, not empty
Author - required, must be an instance of Author class.
Reader
Name, required, String, not empty
Email - required, String, not empty
City - required, String, not empty
Street - required, String, not empty
House - required, Integer, positive
Order
Book - required, must be an instance of Book class.
Reader must be an instance of Reader class
Date must be an instance of a Ruby Date class. The default date is today's date.