Don't install build-essential
It seems to be superfluous and is the source of error in Docker Hub.
Use specyfic tag ubuntu:14.04
Adds NuPIC Walthrough ipython notebook.
Merge pull request #1313 from allanino/issue-1312

Fix broken Docker build
Merge pull request #1316 from scottpurdy/example-notebook

Adds NuPIC Walthrough ipython notebook.