Use active_model_serializer with a non-ActiveRecord object

The model object needs to include it thusly:

# active_model_serializers 0.10+
class ModelName
  include ActiveModel::Serialization
end

# active_model_serializers < 0.10
class ModelName
  include ActiveModel::SerializerSupport
end

This implements the methods needed within the object, and also auto-discovers the serializer matching the object name, so it can be used transparently just like an ActiveRecord object.

This works with active_model_serializers version 0.8.1 through at least 0.9.3.