Fix error when worker dies while regrouping.
Clean up th Job class a bit, provide an empty-response client error.
Major updates to job orchestration and injection.

* Jobs can actually be orchestrated now, with a @job decorator.
* The Client class no longer assumes the function is available and just
  sends a simple JSON payload, the Job class on the server end now handles
  uniqueness and deserialization.

Job functions are now expected to be decorated with the @job decorator.
They can define a standard timedelta delay, or their uniqueness.

The "unique" keyword arg to the @job decorator can be one of:

* False (the default).  This means the job isn't unique at all and any
  queued up will be executed.

* True. This implies that the job's uniqueness is its full arguments.
  The same function called with the same combo of args and kwargs would
  be considered unique, the first enqueueing will go through but
  subsequent ones will be skipped until the first is entirely complete.

* List of strings.  Each string is an argument name.  This behaves the
  same as the "True" value detailed above, but limits which arguments
  are considered when determining when a job queueing is unique.
Bump version to 0.2.0