OperationalError when deferring foreign keys
What database are you using?
 Ha! I was using SQLite (which I usually use when starting something out) but after your response, I just tried it with Postgres and it worked fine. Is this a SQLite limitation, then? Either way, this is helpful since I was going to move this to Postgres eventually anyway.
 SQLite does not support adding constraints. So basically you can follow the documentation example and simply skip the call to add the constraint. I've added a test and updated the docs to make this clear.

I've been trying to use the Proxy() trick, followed by its initialization, then table creation, and finally foreign key creation, exactly as shown in the [Circular foreign key dependencies](https://peewee.readthedocs.org/en/latest/peewee/models.html?highlight=defer#circular-foreign-key-dependencies) documentation. However, I always receive the following error:

``` python
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "/home/dkh/devel/peeweetest/venv/lib/python2.7/site-packages/peewee.py", line 3436, in create_foreign_key
    model_class, field, constraint))
  File "/home/dkh/devel/peeweetest/venv/lib/python2.7/site-packages/peewee.py", line 3339, in execute_sql
    self.commit()
  File "/home/dkh/devel/peeweetest/venv/lib/python2.7/site-packages/peewee.py", line 3185, in __exit__
    reraise(new_type, new_type(*exc_args), traceback)
  File "/home/dkh/devel/peeweetest/venv/lib/python2.7/site-packages/peewee.py", line 3331, in execute_sql
    cursor.execute(sql, params or ())
OperationalError: near "CONSTRAINT": syntax error
```

Wondering if it was with my models or something else on my end, I have tried using the exact models and code on the documentation with the same result. Any thoughts?
