add "is not null" lookup support
I'm not going to merge this since you can accomplish the same using `Q` objects:

```
NullModel.select().where(~Q(char_field__is=None))
```

I added this to the testcase to show how it is possible:
https://github.com/coleifer/peewee/commit/08dabe0d6b82701c48022e5d7a758d29bfaea154
 Fair enough, though you could make the same case for the `ne` operator.
 Q objects don't appear to exist in my version of Peewee. How can I accomplish the same behavior without them? I have models like:

class Owner(Model):
    ...
class Ownable(Model):
    owner = ForeignKeyField(Owner, null=True, default=None)

And I wish to obtain a list of all Ownables which do not have an owner.
 You would do `Ownable.select().where(Ownable.owner >> None)` -- if you want not null: `~(Ownable.owner >> None)`

You have a way to query for columns with NULL values but not NOT NULL values, so I added it.

I wasn't sure if you'd prefer `__isnt` or  `__is_not`, but I went with the latter because it's more explicit.
