Postgres auto field null constraint

References https://code.djangoproject.com/ticket/24042

[The documentation](​https://docs.djangoproject.com/en/dev/howto/custom-model-fields/#emulating-built-in-field-types) on `Field.get_internal_type` implies that it should define custom behavior and ensure that database-correct behavior is followed. For Postgres (though not for sqlite), specifying `get_internal_type()` to be "AutoField" fails to correctly set the field, leading to errors like this:

```pytb
test_auto_field (field_subclassing.tests.CustomField) ... ERROR

======================================================================
ERROR: test_auto_field (field_subclassing.tests.CustomField)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/lucaswiman/opensource/django/tests/field_subclassing/tests.py", line 128, in test_auto_field
    o.save()
  File "/Users/lucaswiman/opensource/django/django/db/models/base.py", line 694, in save
    force_update=force_update, update_fields=update_fields)
  File "/Users/lucaswiman/opensource/django/django/db/models/base.py", line 722, in save_base
    updated = self._save_table(raw, cls, force_insert, force_update, using, update_fields)
  File "/Users/lucaswiman/opensource/django/django/db/models/base.py", line 803, in _save_table
    result = self._do_insert(cls._base_manager, using, fields, update_pk, raw)
  File "/Users/lucaswiman/opensource/django/django/db/models/base.py", line 842, in _do_insert
    using=using, raw=raw)
  File "/Users/lucaswiman/opensource/django/django/db/models/manager.py", line 131, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/Users/lucaswiman/opensource/django/django/db/models/query.py", line 954, in _insert
    return query.get_compiler(using=using).execute_sql(return_id)
  File "/Users/lucaswiman/opensource/django/django/db/models/sql/compiler.py", line 931, in execute_sql
    cursor.execute(sql, params)
  File "/Users/lucaswiman/opensource/django/django/db/backends/utils.py", line 65, in execute
    return self.cursor.execute(sql, params)
  File "/Users/lucaswiman/opensource/django/django/db/utils.py", line 95, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "/Users/lucaswiman/opensource/django/django/db/backends/utils.py", line 65, in execute
    return self.cursor.execute(sql, params)
IntegrityError: null value in column "id" violates not-null constraint
DETAIL:  Failing row contains (null).
```
The attached diff adds a regression test which fails on postgres databases with the error above. I also fixed all other uses of `isinstance(..., AutoField)` I could find in the code.

The immediate motivation for this is writing integration tests which use django-salesforce on a CI system that uses a postgres database. I've filed a separate issue there, though I think fixing the behavior in Django to match the documentation would be a preferable solution. [The package maintainer for django-salesforce agrees](​https://github.com/django-salesforce/django-salesforce/issues/90#issuecomment-67932443).

### How to Reproduce

Check out the master branch and apply my diff from the tests/ directory. Run the tests with a settings file pointing to a postgres database:

```
PYTHONPATH=..:$PYTHONPATH ./runtests.py --verbosity=2 --failfast field_subclassing --settings=test_postgres
```
It will fail with the above stack trace.

### Testing

I verified that the regression test fails without the remainder of the diff. I also verified that the tests still pass on the default test_sqlite file.