Fix build errors

- fix interfaces visibility errors
- fix interfaces properties errors
- add interfaces implementations missings methods
Update IFormAnswer

- Add FindAnswer(...)
- Update AddAnwserFor(...)
Add linked IQuestion in BaseAnswer constructor

- Set empty constructor visibility to private
Add MilitaryForm unit tests
Add FreeQuestion plugin (In progress)
Refactoring model (in progress) | IQuestion have to support IEnumerable
Add nuget package config