Allow users to add curated datasets to list

[bcipolli]

There are lots of nice curated public datasets available (e.g. openfmri.org). It'd be great to see them as available through the website.

Currently, however, there's no way to add such datasets to the website, and it's unclear how to create a notebook with datasets that are not in the system.

It'd be great to have documentation on:

• How to get curated datasets shown at http://datasets.codeneuro.org
• How to use notebooks on datasets not shown on the website.