Can't assign empty list on init
Peewee models perform initialization on startup.  Call the `super` method first:

``` python

def __init__(self):
    super(Foo, self).__init__()
    self.list = []
```

I'm trying to make an attribute of a class an empty list on init, but I get the following error:

```
  File "~/.virtualenv/foobar/lib/python2.7/site-packages/peewee.py", line 281, in __set__
    instance._data[self.att_name] = value
TypeError: 'NoneType' object does not support item assignment
```

Steps to reproduce:

```
class BaseModel(Model):
    class Meta:
        database = database

class Foo(BaseModel):
    bar = CharField()

    def __init__(self):
        self.list = []

x = Foo()
```
