Add nbzip as default to user images

[betatim]

https://github.com/data-8/nbzip

How to deal with all the data and stuff that you might not want to download by default?

[betatim]

If you login to https://hub.earthdatascience.org/staginghub there is now a new icon next to the "upload" button on the far right.

If you click it it will download the current directory as a .tar.gz to your machine. It doesn't bundle up all directories below the current one. If that is something we would find useful I can create a PR and see what the nbzip authors think of it.

There is a (stalled) PR to let people download things as .zip instead of .tar.gz. Seems a bit more userfriendly. Should we poke that PR to get it going again?

[lwasser]

yes!! zip would be much more user friendly. let's poke. if you poke provide a link and i'll poke too!! i used to do that with chris to get things moving :) i'll try this in a bit... still working on my hub build and the docs!!