Authenticating a flask service with JupyterHub

Uses jupyterhub.services.HubAuth to authenticate requests with the Hub in a flask application.

Run

Launch JupyterHub and the whoami service with
```
 jupyterhub --ip=127.0.0.1
```
Visit http://127.0.0.1:8000/services/whoami

After logging in with your local-system credentials, you should see a JSON dump of your user info:

{
 "admin": false,
 "last_activity": "2016-05-27T14:05:18.016372",
 "name": "queequeg",
 "pending": null,
 "server": "/user/queequeg"
}

This relies on the Hub starting the whoami service, via config (see jupyterhub_config.py).

A similar service could be run externally, by setting the JupyterHub service environment variables:

JUPYTERHUB_API_TOKEN
JUPYTERHUB_SERVICE_PREFIX