Change Client class name to Rotterdam.

Once imported, the class "Client" would end up too generic and possibly
conflict with other things.
Big changes to simplify the overall model.

* The Arbiter class is reinstated, we now only have one process
  connecting to redis to pull in jobs at any given time, rather than a
  slew of separate consumers.  In the previous decentrialized model, a
  runaway consumer process could end up errantly consuming jobs without
  permission or control.

* No more per-process concurrency via green threads (for now).  The code
  depended on a release candidate of a third-party library and added all
  sorts of complexity with no clear advantage.  The nature of how the
  system behaves with I/O bound jobs needs to be researched further
  before this feature is reintroduced.

* Fix bug in Payload where a non-string kwarg value would blow up when
  trying to determine job uniqueness.

* Payload objects are no longer passed around the multiprocessing
  queues, just JSON serializations now.

* The number of injector processes is set at two no matter what.  The
  injectors are about as simple as they can get and being able to inject
  two at a time is nice but more is probably not necessary.  Hell, two
  might not even be necessary.  Once the system has stats reporting
  we'll be able to determine the effects of the number of injectors.

* Removed job-related tests as the class has changed significantly since
  written and none of the tests pass.