long double variables have been changed to double.
json: rapidjson library has been added.
json: global rapidjson allocators have been added.
json: function to convert handystats' internal timestamp to system's one has been added.
json: gauge to_json writer has been added.
counter: value_type has been changed to int64_t.
json: counter to_json writer has been added.
json: common json header has been added.
tests: to_json output has been added to counter & gauge tests.