Enable gists for the demo image.

This uses @minrk's notebook extension for sharing as gist.

![screen shot 2014-10-14 at 4 32 15 pm](https://cloud.githubusercontent.com/assets/836375/4638427/60d3e55c-53fa-11e4-86e0-fe998d74b817.png)

We should probably discuss this one before merge, since:

* We're `ADD`ing a remote resource (it's Min's, but still)
* Users should be given a warning about providing a token on a hosted tmpnb server