api: refactor api attribute declarations.

Replace the `method_attributes` and `hidden_attributes` methods with
`api_attributes`. `api_attributes` can be used as a class macro:

    # include only the given attributes.
    api_attributes :id, :created_at, :creator_name, ...

    # include all default attributes plus the `creator_name` method.
    api_attributes including: [:creator_name]

or as an instance method:

    def api_attributes
       [:id, :created_at, :creator_name, ...]
    end

By default, all attributes are included except for IP addresses and
tsvector columns.

app/logical/application_responder.rb

@@ -17,7 +17,7 @@ class ApplicationResponder < ActionController::Responder
       options[:root] ||= resource.table_name.dasherize if resource.respond_to?(:table_name)
     end
 
-    options[:only] ||= params["only"].scan(/\w+/) if params["only"]
+    options[:only] ||= params["only"].scan(/\w+/).map(&:to_sym) if params["only"]
 
     super
   end