Add a test

[minrk]

This is mostly a collection of fixtures to get a test hub running with tokens and permissions.

• runs a real local jupyterhub, with null auth and simplespawner
• configures two service tokens - one with fuller admin permissions, one for the culler
• just one real test that does a basic:
  1. start some servers
  2. cull when they are all fresh (doesn't do anything)
  3. cull again after they should have expired (mock datetime.now into the future to avoid waiting)

closes #36

There are lots of situations to add tests for, but this does the basics, so it should hopefully not be too hard to add them.

Some tests might be more easily served by mocking out the Hub with some pre-fab responses, but that's a bit more complicated, I think.