TDD implementation of book and label methods

[JuanLPalacios]

• Implement methods:
  • add_item method in the Label class
    • should take an instance of the Item class as an input
    • should add the input item to the collection of items
    • should add self as a property of the item object (by using the correct setter from the item object)
  • can_be_archived?() in the Book class
    • should override the method from the parent class
    • should return true if parent's method returns true OR if cover_state equals to "bad"
    • otherwise, it should return false
• Add unit tests for all implemented methods.