Dockerfile for the example notebooks.

This adds a Dockerfile (to be auto-built by the Docker hub), so that someone can bring up an entire environment with (almost) all the dependencies required by the example notebooks. For now I've added `scikit-image`, which is used by one of the widget notebooks.

My plan here, if people like this idea, would be to have a Docker image named `ipython/examples`. Someone would then:

```
docker run -p 8888:8888 ipython/examples
```

To bring up this setup and explore the example notebooks.