Remove PersonOne and PersonOther models

- Use only Person model
- Add allow_blanks param to Person.new to optionally disable validation on
person's income.

```ruby
person_with_blanks = Person.new({}, false);
person_without_blanks = Person.new({
      annual_income: annual_income,
      extra_income: extra_income,
      monthly_net_income: monthly_net_income});
```