Minimal Docker base image

Did some talking with @minrk about how we handle our base Docker images.

We've decided to keep this one minimal and for it to be the foundation for the rest of the images. This Dockerfile will be part of the 3.0 release, which means that we'll probably want to have several tagged versions for people to derive from (in line with our current tagging and branches:

* `ipython/ipython:master`
* `ipython/ipython:3.x`
* `ipython/ipython:rel-3.0.0`

Since 3.0 is not released yet, we'll only have the master tag available. This will give stability to those that would like to work from a point release.