Base url

This sets up a base path so that this jupyter kernel server can be:

* Routed on a proxy (proxy_pass in nginx land)
* Used as part of [tmpnb](https://github.com/jupyter/tmpnb)

To get this running with tmpnb, I used roughly the same setup you had on your demo server and README:

```console
$ docker rm -f $( docker ps -aq)
$ docker build -t odewahn/jupyter-kernel .
$ docker run --net=host -d -e CONFIGPROXY_AUTH_TOKEN=$TOKEN --name=proxy jupyter/configurable-http-proxy --default-target http://127.0.0.1:9999
$ docker run --net=host     -e CONFIGPROXY_AUTH_TOKEN=$TOKEN    -v /var/run/docker.sock:/docker.sock    jupyter/tmpnb    python orchestrate.py --container_ip=0.0.0.0 --image="odewahn/jupyter-kernel" --command="python jupyter-kernel.py --base_path='{base_path}'" --pool_size=5 --redirect-uri="/"
```

The differences here for running it include the use of `base_path` to allow tmpnb to inform this project/server that it will be routed on `base_path`. Additionally, I've set the `redirect-uri` to '/' so that it takes the user to the base page.