Circular references
This is partially fixed by d5a5a86a14b7d631ef8b8b3759bfd8345def9b56 which makes it possible to use `Proxy` with `ForeignKeyField`.  Unfortunately, Postgresql will complain about the `create table` queries generated so I'll need to work around this.
 http://peewee.readthedocs.org/en/latest/peewee/cookbook.html#circular-foreign-key-dependencies

It would be nice to support circular references between tables:

``` python

class User(Model):
    username = CharField()
    last_post = ForeignKeyField(Post, null=True)  # error

class Post(Model):
    author = ForeignKeyField(User, related_name='posts')
    title = CharField()
```

Self-referential FKs are possible using the magic string `'self'` but this is not possible between tables.
