Implement __ne__ for CompositeKey.
Wouldn't it just be:

```python
def __ne__(self, other):
    return ~(self == other)
``` Yeah, that works too. Forgot about `~` when I wrote it.
For a model with a single primary key, you can do both of these:

```python
(obj._meta.primary_key == obj._get_pk_value())
(obj._meta.primary_key != obj._get_pk_value())
```

And both return a `peewee.Expression` object.

However if the primary key happens to be CompositeKey, the `!=` compare returns `False`.

This fixes that. I can write tests if you'd like.