Added a south_field_triple method so south knows how to deal with a SortedManyToManyField.
Added a south_field_triple method so south knows how to deal with a SortedManyToManyField.

fabrique/django-sortedm2m,gregmuellegger/django-sortedm2m,gregmuellegger/django-sortedm2m,gradel/django-sortedm2m,fabrique/django-sortedm2m,gregmuellegger/django-sortedm2m,fabrique/django-sortedm2m,gradel/django-sortedm2m,MathieuDuponchelle/django-sortedm2m,MathieuDuponchelle/django-sortedm2m,gradel/django-sortedm2m