Implement setTaskOption on VTask

We want to a periodic task's "interval" option based on a multiple of
another task's option. Get this error w/out this change.

ERROR:TestService.SetOptionTask:Unhandled exception in SetOptionTask
Traceback (most recent call last):
  File "/Users/joeldodge/joel_sparts/sparts/vtask.py", line 100, in _run
    self._runloop()
  File "/Users/joeldodge/joel_sparts/sparts/tasks/periodic.py", line 54, in _runloop
    self.execute()
  File "/Users/joeldodge/joel_sparts/tests/tasks/test_settaskoption.py", line 17, in execute
    self.some_option += self.INCREMENT
  File "/Users/joeldodge/joel_sparts/sparts/sparts.py", line 90, in __set__
    self._setter(obj)(self.name, value)
  File "/Users/joeldodge/joel_sparts/sparts/sparts.py", line 103, in _setter
    assert setter is not None
AssertionError: assert None is not None