Only load generators when needed

- use hook_for to hook in the serializer and remove load_generators
- move generators so they can be found by rails
- move to_prepare block to railtie config

This commit improves the way the generators are loaded and how
they extend the resource generator.

* The initializer block has been changed to a `generator` block which is only executed when generators are needed.
* The call to `app.load_generators` has been removed. There is no need to load *all* generators.
* The `resource_override.rb` has been changed to use `hook_for` to extend the resource generator.
* The directory for the generators has been moved to match the way Rails looks to load generators.

With `hook_for` it would now be possible for a user to pass `--no-serializer` to skip that option.
The `--serialize` option also now shows up in the generator help with `rails g resource --help`.

These changes follow the way the Draper gem extends the `controller` generator.

lib/generators/rails/USAGE

@@ -0,0 +1,6 @@
+Description:
+    Generates a serializer for the given resource.
+
+Example:
+    `rails generate serializer Account name created_at`
+

lib/generators/rails/resource_override.rb

@@ -0,0 +1,10 @@
+require 'rails/generators'
+require 'rails/generators/rails/resource/resource_generator'
+
+module Rails
+  module Generators
+    class ResourceGenerator
+      hook_for :serializer, default: true, boolean: true
+    end
+  end
+end

lib/generators/rails/serializer_generator.rb

@@ -0,0 +1,36 @@
+module Rails
+  module Generators
+    class SerializerGenerator < NamedBase
+      source_root File.expand_path('../templates', __FILE__)
+      check_class_collision :suffix => 'Serializer'
+
+      argument :attributes, :type => :array, :default => [], :banner => 'field:type field:type'
+
+      class_option :parent, :type => :string, :desc => 'The parent class for the generated serializer'
+
+      def create_serializer_file
+        template 'serializer.rb.erb', File.join('app/serializers', class_path, "#{file_name}_serializer.rb")
+      end
+
+      private
+
+      def attributes_names
+        [:id] + attributes.reject(&:reference?).map! { |a| a.name.to_sym }
+      end
+
+      def association_names
+        attributes.select(&:reference?).map! { |a| a.name.to_sym }
+      end
+
+      def parent_class_name
+        if options[:parent]
+          options[:parent]
+        elsif defined?(::ApplicationSerializer)
+          'ApplicationSerializer'
+        else
+          'ActiveModel::Serializer'
+        end
+      end
+    end
+  end
+end

lib/generators/rails/templates/serializer.rb.erb

@@ -0,0 +1,8 @@
+<% module_namespacing do -%>
+class <%= class_name %>Serializer < <%= parent_class_name %>
+  attributes <%= attributes_names.map(&:inspect).join(", ") %>
+<% association_names.each do |attribute| -%>
+  has_one :<%= attribute %>
+<% end -%>
+end
+<% end -%>