* JSON visitor are located in the format namespace
* Visitor are *real* visitor
* Visitable class has been renamed to Object
* Due to a bug in Visual Studio 2015, we move to VS2017
API changes:
- 'has_flag' renamed to 'has'
- 'add' to add a flag - added
- 'remove' to remove a flag - added
- operator+= to add a flag - added
- operator-= to remove a flag - added