Add support for IS NOT operator

Allows for doing

```php
$query->where(['field IS NOT' => null]);
```
For the record, you could just cache the comparison:

```php
$valueIsNotNull = $value !== null;
if ($operator === 'is' && $valueIsNotNull) {
    $operator = '=';
}

if ($operator === 'is not' && $valueIsNotNull) {
    $operator = '!=';
}
```

:P