Add many_to_native method

Allows serializers to be easily customized when many=True rather than always returning a list of objects.

In our case, we wanted to replicate Stripe's api which returns a similar format to:

```json
{
  "object": "customer",
  "id": "cus_2o1yHp2LqS4yoo",
  "cards": {
    "entity": "list",
    "url": "/v1/customers/cus_2o1yHp2LqS4yoo/cards",
    "data": [
      {
        "id": "card_2o1yR1PV91qREi",
        "object": "card",
        "last4": "4242",
        "type": "Visa",
      }
    ]
  }
}
```

Note that "cards" is an object with "entity", "url" and "data" keys rather than an array.

With this pull req, we can now define our own `many_to_native` method on a serializer superclass. Defining it this way allows top level and all nested lists to be presented in the same way. I looked at other ways of accomplishing the same thing, but this seemed like the cleanest (though I'm fairly new to DRF).

```python
def many_to_native(self, objects):
    data = {
        'entity': 'list',
        'url': self.get_list_url(objects),
        'data': [self.to_native(item) for item in objects]
    }

    return data
```