improve south support

(is there a way for me to mark this as a whishlist bug?)

south is a widely used database migration framework for django. It would be nice if sortedm2m would support south out-of-the-box. This would involve adding a bit of code to fields.py (introspection rules south uses), and possibly fixing bugs.

1) south needs introspection rules (see http://south.aeracode.org/docs/customfields.html#extending-introspection), the following appear to work (with south 0.7.1 and trunk version of sortedm2m):
---------------------------------------
try:
    from south.modelsinspector import add_introspection_rules
    add_introspection_rules([
        (
          [SortedManyToManyField],
          [],
          {'sorted': ['sorted', {'default': True}],
           },
        ),
        ], ["^sortedm2m\.fields\.SortedManyToManyField"])
except ImportError:
    pass
---------------------------------------
I think the best place to put this would be the end of fields.py

2) The following models.py works with the introspection rules above:
------------------------------
    class Book(models.Model):
        title = models.CharField(max_length=50)

    class Shelf(models.Model):
        books = SortedManyToManyField(Book)
------------------------------

However, putting the two models in reverse order, like this:
------------------------------
    class Shelf(models.Model):
        books = SortedManyToManyField('Book')

    class Book(models.Model):
        title = models.CharField(max_length=50)
------------------------------

Triggers the following errors when the migation is run:
------------------------------
Error: One or more models did not validate:
myapp.shelf: 'books' has an m2m relation with model <class 'myapp.models.Book'>, which has either not been installed or is abstract.
myapp.shelf: 'books' specifies an m2m relation through model <class '.myapp_Shelf_books'>, which has not been installed.
myapp.shelf: Accessor for m2m field 'books' clashes with related m2m field 'Book.shelf_set'. Add a related_name argument to the definition for 'books'.
myapp.shelf: 'books' has an m2m relation with model <class 'myapp.models.Book'>, which has either not been installed or is abstract.
myapp.shelf: 'books' specifies an m2m relation through model <class '.myapp_Shelf_books'>, which has not been installed.
myapp.shelf: The model Shelf has two manually-defined m2m relations through the model myapp_Shelf_books, which is not permitted. Please consider using an extra field on your intermediary model instead.
myapp.shelf: Accessor for m2m field 'books' clashes with related m2m field 'Book.shelf_set'. Add a related_name argument to the definition for 'books'.
------------------------------

This might be caused by a problem in south, I didn't investigate further. I did, however try an other workaround for this (reordering the model classes like in the first, working, example being one possibility): First creating a migration which adds the Book model, and adding the Shelf model in a second migration. In this case, I got the following exception when the migration was created:
------------------------------
$ python manage.py schemamigration myapp add_shelf --auto
<class 'myapp.models.Book'> <sortedm2m.fields.SortedManyToManyField object at 0x18e8390> <class 'myapp.models.Shelf'> books
<class 'myapp.models.Book'> <sortedm2m.fields.SortedManyToManyField object at 0x18e8390> <class 'myapp.models.Shelf'> books
 + Added model myapp.Shelf
Traceback (most recent call last):
  File "manage.py", line 11, in <module>
    execute_manager(settings)
  File "/mnt/data/abeld/WORK/maven/FirmNetOnline/sortedm2m_bug/env/lib/python2.6/site-packages/django/core/management/__init__.py", line 438, in execute_manager
    utility.execute()
  File "/mnt/data/abeld/WORK/maven/FirmNetOnline/sortedm2m_bug/env/lib/python2.6/site-packages/django/core/management/__init__.py", line 379, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/mnt/data/abeld/WORK/maven/FirmNetOnline/sortedm2m_bug/env/lib/python2.6/site-packages/django/core/management/base.py", line 191, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "/mnt/data/abeld/WORK/maven/FirmNetOnline/sortedm2m_bug/env/lib/python2.6/site-packages/django/core/management/base.py", line 218, in execute
    output = self.handle(*args, **options)
  File "/mnt/data/abeld/WORK/maven/FirmNetOnline/sortedm2m_bug/env/lib/python2.6/site-packages/south/management/commands/schemamigration.py", line 143, in handle
    action.add_forwards(forwards_actions)
  File "/mnt/data/abeld/WORK/maven/FirmNetOnline/sortedm2m_bug/env/lib/python2.6/site-packages/south/creator/actions.py", line 29, in add_forwards
    forwards.append(self.forwards_code())
  File "/mnt/data/abeld/WORK/maven/FirmNetOnline/sortedm2m_bug/env/lib/python2.6/site-packages/south/creator/actions.py", line 428, in forwards_code
    "left_field": self.field.m2m_column_name()[:-3], # Remove the _id part
TypeError: 'NoneType' object is unsubscriptable
------------------------------