NoneType error when deleting an instance with multiple null foreign keys
Pushed a testcase... It passed locally on py2.7 with sqlite, so we'll see what's up.
 I found what's happening. My model has a nullable 'delete' field and that overwrites the delete method. 
Thank you for prompt reply but it was my distraction :)

You could consider renaming the delete method to _delete, but I do not know whether this would make sense with peewee.

peewee raises a NoneType error when deleting an instance with multiple null foreign keys, e.g.:

```
Schools(Base):
    id = PrimaryKeyField(primary_key=True)
    name = CharField()

Parent(Base):
    id = PrimaryKeyField(primary_key=True)
    name = CharField()

Children(Base):
    id = PrimaryKeyField(primary_key=True)
    parent = ForeignKeyField(Parents, null=True)
    school = ForeignKeyField(Schools, null=True)
    age = IntegerField()
```

Then

```
somechild = Children(age=12)
somechild.save()
...
somechild.delete_instance()
```

Will result in this:

```
 def delete_instance(self, recursive=False, delete_nullable=False):
    if recursive:
        dependencies = self.dependencies(delete_nullable)
        for query, fk in reversed(list(dependencies)):
            model = fk.model_class
            if fk.null and not delete_nullable:
                model.update(**{fk.name: None}).where(query).execute()
            else:
                model.delete().where(query).execute()
    >return self.delete().where(self._pk_expr()).execute()
    TypeError: 'NoneType' object is not callable
```

I was going to post this on Stackoverflow, but then I've seen that getattr(somechild, 'delete') returns None.

I am using peewee 2.8.0 on python 3.4.3
