tweaks to url checking

- update to be aware of new (and old and private) gist urls
- stop checking .ipynb file extension on raw urls
- avoid 500 errors on invalid notebooks at raw URLs

This branch can be tested at http://nbtest.herokuapp.com

Also, pinging @Carreau again about why we have a master nbviewer that is different from what we are actually using.  I propose simply clobbering current master with the heroku branch.