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 #157 from jiffyclub/patch-1

Change open-in-nbviewer link
option for MockCache
accept seding content as gzip
extend no-cache to jinja templates
update readme
Merge pull request #158 from Carreau/gzip-no-cache

Gzip no cache
added local install section to README
added txt extension
use headroom to remove header on scroll
force add necesary file of headroom
add font awesome
add anmate.css
stick bar on top on mobiles
add fontawesome link and localfilehandler
no need to fade bar anymore
Merge pull request #160 from yaph/readme-install

added local install section to README
Merge pull request #154 from Carreau/style-refresh

Style refresh
jquery and nowrap
Add travis, simple invoke tasks.