Add support for multiple filters on the same field.

Allow things like getting all the articles created between 2 dates.

```
curl https://www.example.org/api/articles?filter[created][value][0]=12345&filter[created][operator][0]=">="&filter[created][value][1]=67890&filter[created][operator][1]="<="
```

Maybe the following is clearer.

```php
$request['filter'] = array(
  'created' => array(
    'value' => array(12345, 67890),
    'operator' => array('>=', '<='),
    'conjuction' => 'AND',
  ),
);
```