Enable gists for the demo image.

This uses @minrk's notebook extension for sharing as gist.

![screen shot 2014-10-14 at 4 33 20 pm](https://cloud.githubusercontent.com/assets/836375/4638440/88015754-53fa-11e4-80f9-dc8204c9f00e.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
* Liabilities?