Make Adapters registerable so they are not namespace-constrained

Changes:

- Introduce Adapter::get for use by Serializer.adapter
- Move Adapter-finding logic from Adapter::adapter_class into Adapter::get

Introduced interfaces:

- non-inherited methods
```ruby
ActiveModel::Serializer::Adapter.adapter_map     # a Hash<adapter_name, adapter_class>
ActiveModel::Serializer::Adapter.adapters        # an Array<adapter_name>
ActiveModel::Serializer::Adapter.register(name, klass) # adds an adapter to the adapter_map
ActiveModel::Serializer::Adapter.get(name_or_klass)    # raises Argument error when adapter not found
```

- Automatically register adapters when subclassing

```ruby
      def self.inherited(subclass)
        ActiveModel::Serializer::Adapter.register(subclass.to_s.demodulize, subclass)
      end
```

- Preserves subclass method `::adapter_class(adapter)`

```ruby
      def self.adapter_class(adapter)
        ActiveModel::Serializer::Adapter.get(adapter)
      end
```

- Serializer.adapter now uses `Adapter.get(config.adapter)` rather than have duplicate logic

test/adapter_test.rb

@@ -19,16 +19,6 @@ module ActiveModel
         assert_equal @serializer, @adapter.serializer
       end
 
-      def test_adapter_class_for_known_adapter
-        klass = ActiveModel::Serializer::Adapter.adapter_class(:json_api)
-        assert_equal ActiveModel::Serializer::Adapter::JsonApi, klass
-      end
-
-      def test_adapter_class_for_unknown_adapter
-        klass = ActiveModel::Serializer::Adapter.adapter_class(:json_simple)
-        assert_nil klass
-      end
-
       def test_create_adapter
         adapter = ActiveModel::Serializer::Adapter.create(@serializer)
         assert_equal ActiveModel::Serializer::Adapter::FlattenJson, adapter.class