Add support for Kerberos Local Authenticator

[dvalters]

Supports creating local system user accounts on login to a jupyterhub server. (In a similar fix to: jupyterhub/ldapauthenticator#36, for LDAPAuthenticator.)

Closes #5

To use the feature, these options are added/changed in the jupyterhub_config.py file:

c.JupyterHub.authenticator_class = 'kerberosauthenticator.KerberosLocalAuthenticator'
c.LocalAuthenticator.create_system_users = True

[jcrist]

Would you mind adding this to the docs? Would just need to add another line with this class to docs/source/options.rst.

Configuration Options
=====================

.. autoconfigurable:: kerberosauthenticator.KerberosAuthenticator

.. autoconfigurable:: kerberosauthenticator.KerberosLocalAuthenticator

[jcrist]

Looks like the tests aren't passing due to changes to JupyterHub's testing infrastructure in the latest release (had a similar problem in https://github.com/jcrist/yarnspawner). Don't worry about the tests, I'll figure those out. Everything else looks good.

[dvalters]

Sure, I'll update the docs as well

[jcrist]

Thanks, merging.