add feature to export dictionaries in export_as_csv()

is now possible to pass dictionaries to export_as_csv(), so
it is possible put joined fields in the exported data

 fields = ['name', 'linkedtable__fieldname']
 qs = Model.objects.values(*fields)
 export_as_csv(qs, fields)
add feature to use callable as 'columns' in export_as_csv()