[FIX] calendar: correct search in recurrent event

Add a 3 use cases with tests that were failing before.

datetime filters were not working correctly on stop filters
The `_get_recurrent_date_by_event` was only computing recurrency on the start
field of an event which can make a difference. A recurring event
  start=2017-01-01 09:00:00; stop=2017-01-01 10:00:0
was not matched in a search ('stop', '>', '2017-01-01 09:30:00')

The filter != was not handled in the recurring use cases.
e.g. making the search 'start is set' in the web client is equal to
  ('start', '!=', False)

Searching with only a start filter matching recurring events but not the
first ("real") occurence, was returning no result if no stop filter was present.
  start=2017-01-01 09:00:00; count=5
had no result in a search ('start', '>', '2017-01-02')