1.04 nav item auto-disappear is done.

Why:
In order to improve UX when user clicks nag items and window scrolls
down in small size device.
How:
 1. find the threshold width for small device in media query.css (768px)
2. find window width($(window).width())
3. set up condition(if) to make nav item disappear  after scrolling.
What:
when user click nag items in small device, the nav items will slide up
after clicking. When user click nag items in big device, everything
keeps originnal.