Feature/adapter

Adapter configuration using AMS.config. It may be configured with a class or symbol:

    ActiveModel::Serializer.config.adapter  =  MyAdapter

    # or

    ActiveModel::Serializer.config.adapter  =  :json_api

Start to work on JsonApi adapter to better understand interface between adapters and serializers.
Rename null adapter to json adapter. Null adapter always return empty hash now.
Introduce ArraySerializer.
Add a lot of tests.

I would be glad to receive any feedback.