Methods for unicode sequences conversion

This methods are mostly needed when saving some unicode data into JSON.
E.g. `json_encode(array('Тестовая строка'))` returns `["\u0422\u0435\u0441\u0442\u043e\u0432\u0430\u044f \u0441\u0442\u0440\u043e\u043a\u0430"]`