allow store to deserialize polymorphic relationships

With this change, no need to change the JSON structure to have all objects at the root, with referential ids in the value of the parent attributes.

I feel this is helpful in the fact that you don't need to create extra serializers to overwrite the extractSingle function in order to change traditional JSON to what Ember-data expects.  However, if you prefer to create specific serializers that will tranform your payload, this is a non-breaking change.