Authenticating a service with JupyterHub

Uses jupyterhub.services.HubAuthenticated to authenticate requests with the Hub.

There is an implementation each of cookie-based HubAuthenticated and OAuth-based HubOAuthenticated.

Run

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

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 services, via config (see jupyterhub_config.py).

You may set the hub_users configuration in the service script to restrict access to the service to a whitelist of allowed users. By default, any authenticated user is allowed.

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

JUPYTERHUB_API_TOKEN
JUPYTERHUB_SERVICE_PREFIX

or instantiating and configuring a HubAuth object yourself, and attaching it as self.hub_auth in your HubAuthenticated handlers.