Field allow null by default
Thanks for the suggestion -- I am going to stick with the current behavior, though.

Hi, when initializing (create) record; sometimes; mostly we have to leave other non indexed fields with default value or null for future fill.

May I suggest you to allow null by default, it's rare of important field to be not null

``` python
class Field(object):
    def __init__(self, null=True, db_index=False, unique=False, verbose_name=None,
                 help_text=None, *args, **kwargs):
```

null=**True**

Thanks
