Create classes for 'label' and 'milestone' objects? Or not?

[dongskyler]

Both 'label' and 'milestone' classes should be extended from a common class.

Use simple JSON to store data (labels and milestones)? Or create classes for them?

Perhaps, just stick with JSON. Creating classes add a lot of complexities. Besides, those instances of aforementioned classes won't have any methods anyway, as this API is designed with a functional programming spirit. All (or almost all) functions in this repo are pure (without side effects). Let's keep those functions pure.