Default to Python 3

In support of #8.

Right now I've just installed it and I'm trying to understand the right way to default to python3. There is the option of symlinking both `python` and `pip`.
Side note: The notebook and scipyserver images are not intended to be used with the spawners of the multiuser notebook server as was discussed last week. SSL termination, authentication, etc. would all get handled by the Jupyter Hub and the running notebook server would be a vanilla server.