function setActiveElement(slides, currentIndex) {
        var _options = options,
            classNameActiveSlide = _options.classNameActiveSlide;


        slides.forEach(function (element, index) {
            if (element.classList.contains(classNameActiveSlide)) {
                element.classList.remove(classNameActiveSlide);
            }
        });

        slides[currentIndex].classList.add(classNameActiveSlide);
    }
private
set active class to element which is the current slide