Error handling, bugfixes, code revamp

• Fixed working directory issue on Unix-like systems
• Added new error handling functions: json_get_last_error(), json_get_last_error_extra(), json_get_error_extra_string(errno, dst[] , const len = sizeof(dst)), json_get_error_string(errno, dst[], const len = sizeof(dst))
• Revamped some code to take advantage of modern C++

Bugfixes, string parsing

• Added new function json_parse_string
• Renamed json_open to json_parse_file
• Fixed memory leak in json_get_string
• Fixed crash if invalid file path is passed to json_parse_file

Initial release - parser

There's plenty of work to be done, but the the parser is nearly finished.