json_dump: public methods 'fill()' and 'to_string()' added that allows to fill in rapidjson::Value from metrics map and create std::string with json dump from metrics map.
tests: json_dump test added.
config: default metrics_dump parameters changed (interval = 750ms, to-json = false).
One can create json dump from metrics dump as follows: handystats::json_dump::to_string(*HANDY_METRICS_DUMP()).