Adds "Any" option to AllValuesFilter

This adds the "Any" option to AllValuesFilter, as asked on #36 and mentioned on the roadmap (#75).

Used the option from `django.db.models.fields.BLANK_CHOICE_DASH`, that will be [translated to _("All")](https://github.com/alex/django-filter/blob/develop/django_filters/widgets.py#L63) if using the LinkWidget