Code style.
More convenient naming.
Json formatter config now looks more cleaner and clearer.
Refactoring done over json formatter config parser.
Register json formatter in factory.
Added logstash example.
One task less.
More comments with logstash example.

Fixed json tree building.
Default example has been renamed to trivial.
Value mapping now lays in its proper place.
Meaningless styling.
Added forgotten include.
Attribute value mapper can now accepts both packed and extracted values.
Custom attribute value mapping added to json formatter.
Minor decomposition.
Using list instead of vector to keep mapped valued alive until `json_visitor_t` lives itself.
Attribute values mapper can be assigned now to all formatter objects.

Fixed memory corruption bug in json formatter. Using std::list instead of std::vector to keep mapped valued alive until `json_visitor_t` lives itself.
Logstash example now maps timestamp and level values.
Using boost::optional instead of std::tuple for returning possibly uninitialized values.
Tiny clean up.