Fix problem with db_field in distinct()

Hello,

I Fix problem with db_field in distinct(), Issue: https://github.com/MongoEngine/mongoengine/issues/260

mongoengine/queryset.py 

(-)        return self._dereference(self._cursor.distinct(field), 1,
(-)                                 name=field, instance=self._document)
(+)
(+)        try:
(+)            field_document = QuerySet._lookup_field(self._document, field) 
(+)            if field_document:
(+)                field = field_document[0].db_field
(+)        finally:
(+)            return self._dereference(self._cursor.distinct(field), 1,
(+)                                     name=field, instance=self._document)