Implement test scheduling interface.
Implement test scheduling interface.

  - Subclasses of TestScheduler.
  - Important methods: next(), stop(), close_old_circuits()
  - Implemented HammerScheduler, (currently) the default scheduler for TorBEL.
     - Continually hammers the Tor network, testing until you tell it to stop.
  - Placeholder for ConservativeScheduler, suggested by Sebastian.

hbock/torbel