fix bug #228, part 1

close part of bug #228
Instead of this if-else:
```python
member_criteria = dict(current_party__isnull=False,start_date__gte = ranges[0][0])
if ranges[0][1] is not None: member_criteria['start_date__lte'] = ranges[0][1]
mk_ids = Member.objects.filter(**member_criteria).values_list('id',flat=True)
```