Allow distinct in DateRangeFilter
Allow distinct in DateRangeFilter

Example: filtering for items sold in the past 7 days etc.

```saleitem__sale__datetime = df.DateRangeFilter(label='Date sold', distinct=True)```

newfylox/django-filter,thenewguy/django-filter,jpadilla/django-filter,thenewguy/django-filter,sbellem/django-filter,jpadilla/django-filter,alex/django-filter,newfylox/django-filter,alex/django-filter,z0by/django-filter,sbellem/django-filter,z0by/django-filter