[WIP] Spec out volumes for containers.

Borrowed straight from https://github.com/jupyter/dockerspawner/pull/5, committed via @ssanderson with a bit of changes on our `ContainerConfig`. Still need to make this configurable on tmpnb launch.
Cool.  Would you want to do a hangout some time this week to talk about what your wants/needs/design thoughts?  I think I'm also going to sit in on the IPython design meeting this week if it would make sense to talk about it then.