create a cron scenario.

2015-10-15 15:12:27,118 domogik-scenario INFO Create or save scenario : name = '0', id = '0', json = '{"type":"dom_condition","id":"1","IF":{"type":"cron.CronTest","id":"30","cron.cron":"0 15 15 ? * MON-FRI"},"DO":{"type":"call_url.CallUrlAction","id":"36","url":"http://ojn.tikijs.dyndns.org/ojn/FR/api.jsp?sn=0013d382e743&token=3436ea85b255ab81d404f87fe32a0125&tts=il est 15 heure 15."},"deletable":false}'
2015-10-15 15:12:27,217 domogik-scenario DEBUG Create action instance call_url.CallUrlAction with uuid d609a67c-6931-4bd1-9b3e-b1119cf60d1a
2015-10-15 15:12:27,217 domogik-scenario DEBUG Create test instance cron.CronTest with uuid ea21ac17-918f-4a01-8e3f-a0b154f212f4
2015-10-15 15:12:27,218 domogik-scenario WARNING Trigger <bound method ScenarioInstance.generic_trigger of <domogik.scenario.scenario.ScenarioInstance instance at 0x2d52950>> called by parameter <domogik.scenario.parameters.cron.CronParameter instance at 0x2d3c290>
2015-10-15 15:12:27,218 domogik-scenario ERROR Exception occured during message processing.
2015-10-15 15:12:27,218 domogik-scenario DEBUG get_parsed_condition called but parsed_condition is empty, try to parse condition first
2015-10-15 15:12:27,220 domogik-scenario DEBUG Traceback (most recent call last):
  File "/home/domogik/src/domogik/bin/scenario.py", line 109, in on_mdp_request
    payload = mapping[msg.get_action().split('.')[0]][msg.get_action().split('.')[1]](**msg.get_data())
  File "/home/domogik/src/domogik/scenario/manager.py", line 140, in update_scenario
    self.create_scenario(name, json_input, cid, dis, desc, True)
  File "/home/domogik/src/domogik/scenario/manager.py", line 192, in create_scenario
    scen = ScenarioInstance(self.log, cid, name, payload, dis)
  File "/home/domogik/src/domogik/scenario/scenario.py", line 96, in __init__
    self._instanciate()
  File "/home/domogik/src/domogik/scenario/scenario.py", line 142, in _instanciate
    self._parsed_condition = self.__parse_if_part(self._json['IF'], datatypes)
  File "/home/domogik/src/domogik/scenario/scenario.py", line 201, in __parse_if_part
    test[0].fill_parameters(part)
  File "/home/domogik/src/domogik/scenario/tests/abstract.py", line 146, in fill_parameters
    self._parameters[test].fill(param)
  File "/home/domogik/src/domogik/scenario/parameters/cron.py", line 69, in fill
    self.expr = CronExpression(params["cron"])
  File "/home/domogik/src/domogik/common/cron.py", line 112, in __init__
    self.compute_numtab()
  File "/home/domogik/src/domogik/common/cron.py", line 155, in compute_numtab
    unified.update(self._parse_atom(cron_atom, span))
TypeError: 'NoneType' object is not iterable