support for a demo container

[johrstrom]

This adds initial support for a demo container. The idea is simple - Lot's of folks need a way to "take a test drive" of OnDemand. This solves that problem.

It's meant to be an all in 1 container that "just works" out of the box with no need for configuration or to read the documentation. You fire it up and you can just use it to explore what OnDemand is and does.

• user is already provided (username jessie@localhost, password owens)
• Jupyter app is already provided (I had to write a localhost adapter lol

You build/start it through the rake task rake demo:start. Ultimately this would go to Dockerhub so folks could just pull it from there.

Other things we should do later

• fix the shell app
• write a MOTD
• see if we can support a desktop