An attempt to make a Hub placeholder that only Authenticate the users.

This is 3 things:

• A Phony spawner that does nothing,
• A Proxy Authenticator that wrap set authenticators to intercept the set_login_cookie to set a cookie for the proxy to know to which hub to dispatch.

A strip down version of the JupyterHub application that does not do some stuff (like services etc... but this appear to be a rat's nest and we've reimplemented half if not more of it), so having that as a JupyterHub sub (or super) class is not out of question.