responsive navbar

The navbar now is responsive. Custom javascript is added to highlight
the active navtab. (However it's less blue now)

modified:   _includes/banner.html
modified:   _includes/header.html
modified:   _includes/javascript.html
modified:   _layouts/people.html
modified:   css/blog.css
modified:   people/index.html

Signed-off-by: Yishen Miao <mys721tx@gmail.com>