Change open-in-nbviewer link

nbviewer has been linking to my blog post about the open-in-nbviewer bookmarklet, but I also maintain browser extensions that people may be interested in. This updates the navbar to link to the Open-in-nbviewer project page, which includes the bookmarklet and browser extensions.
Merge pull request #152 from Carreau/cleanup

cleanup nbviewer frontpage
Merge pull request #157 from jiffyclub/patch-1

Change open-in-nbviewer link
use headroom to remove header on scroll
add headroomjs
add headroomjs
add font awesome
add anmate.css
stick bar on top on mobiles + option for MockCache
add fontawesome link and localfilehandler
no need to fade bar anymore
accept seding content as gzip