Adds support for polymorphic has_many associations

This adds support for has_many polymorphic associations to active_model_serializers.

For an ```Attachment``` model with many ```Attachable``` models associated, where an ```Email``` extends the ```Attachable``` model,  the output would be something like this:

```
{
  "attachment":  {
     "name": 'logo.png',
      "url": 'http://example.com/logo.png',
      "attachable_ids": [{
         "type": "email",
          "id": 1
       }]
    },
    "emails": [{
        "id": 1,
        "subject":  "foo",
        "body": "bar"
    }]
}
```