djeneralize.fields.SpecializedForeignKey is not supported by South

[rhunwicks]

If you attempt to use South migrations with djeneralize.fields.SpecializedForeignKey you get an error:

./manage.py schemamigration appname --auto
 ! Cannot freeze field 'appname.modelname.fieldname'
 ! (this field has class djeneralize.fields.SpecializedForeignKey)

 ! South cannot introspect some fields; this is probably because they are custom
 ! fields. If they worked in 0.6 or below, this is because we have removed the
 ! models parser (it often broke things).
 ! To fix this, read http://south.aeracode.org/wiki/MyFieldsDontWork