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