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

app/logical/reports/upload_tags.rb

@@ -1,32 +1,13 @@
 module Reports
   class UploadTags < ::Post
-
     def readonly?
       true
     end
 
-    module ApiMethods
-
-      def as_json(options = {})
-        options ||= {}
-        options[:only] ||= [:id]
-        super(options)
-      end
-
-      def to_xml(options = {}, &block)
-        options ||= {}
-        options[:only] ||= [:id, :uploader_id]
-        super(options, &block)
-      end
-
-      def method_attributes
-        [:uploader_tags, :added_tags, :removed_tags]
-      end
-
+    def api_attributes
+      [:id, :uploader_id, :uploader_tags, :added_tags, :removed_tags]
     end
 
-    include ApiMethods
-
     def uploader_tags_array
       @uploader_tags ||= begin
         uploader_versions = versions.where(updater_id: uploader_id)