Remove useless typename

Avoid a warning with GCC 4.9:

son.hpp:2361:49: warning: declaration 'struct std::bidirectional_iterator_tag' does not declare anything
         using iterator_category = typename std::bidirectional_iterator_tag;
                                                 ^

Avoid warning with Xcode's clang

94de11e, f673c8d, 540b763

 Fix typos

Replace `default_callback` function with `nullptr` and check for null…

Replace `default_callback` function with `nullptr` and check for null callback function before calling it. This is cleaner and better performing.

support enum

construct enum type to basic_json (proxy by int type)
```
    enum { t = 0 };
    json  j = json::array();
    j.push_back(t);

    j.push_back(json::object({
         {"game_type", t}
    }));
```