[2pt] (Team member #1) Create book & Label class

[abel-tefera]

• Create a Bookclass in a separate .rb file.
• Create a Labelclass with an association to the Item class (in a separate .rb file).
• All Book class properties visible in the diagram should be defined and set up in the constructor method.
• All Label class properties visible in the diagram should be defined and set up in the constructor method.