As I learned in https://github.com/melpon/wandbox/issues/209, this library is already installed at Wandbox, so we need to adjust the examples to use `#include "json.hpp"` insteas of `#include <json.hpp>`.
Replacing references to std exceptions with user-defined exceptions. Also changed some examples to the new exceptions.
Same as get_ptr() but for references. If the type is incompatible it throws (get_ptr() returns null). Implemented in terms of get_ptr().
