Add 'start' and 'end' parameters to the ScheduleVoter

[debreczeniandras]

Instead of the one parameter 'schedule' the voter should be configurable to accept 'from' and 'end' date times.
At least one of these parameters should be required.
If only 'end' is defined, 'start' should be treated as NOW.

If both parameters defined, start must be lower than end.