cJSON C++ wrapper

This MR makes calibration files be saved and loaded in json format (v2) instead of binaries. The old v1 binary format will be used if no .json extension is found in the calibration file.

This was originally implemented with cv::FileStorage and that's why there is a commit with that version but, considering it might be better to not depend on OpenCV, a cJSON C++ wrapper with similar functionality to cv::FileStorage was implemented in u_json.hpp and used instead.

It might be a good idea to have a json schema in place soon for the file format once it gets stable.