Grape formatter feature requested in #1258

- adds handling for when the returned resource is not serializable via ams
 - fix for when resource is an Array
 - Moves grape include to grape namespace. Changes Enumerable to Array because a plain hash is enumerable.
 - Add integration test
 - Refine scope of Grape version dependency
 - Assert that the response is equal to a manually defined JSON string
 - Add single module to include in Grape projects
 - Create a Serializable Resource to test rails-api from Grape
 - Update docs
 - Fix discrepency between ActiveRecord 4.0 - 4.1 and 4.2
 - Updated Changelog
 - Remove parens from `render`, use `serializable` in all tests.

lib/grape/active_model_serializers.rb

@@ -0,0 +1,14 @@
+# To add Grape support, require 'grape/active_model_serializers' in the base of your Grape endpoints
+# Then add 'include Grape::ActiveModelSerializers' to enable the formatter and helpers
+require 'active_model_serializers'
+require 'grape/formatters/active_model_serializers'
+require 'grape/helpers/active_model_serializers'
+
+module Grape::ActiveModelSerializers
+  extend ActiveSupport::Concern
+
+  included do
+    formatter :json, Grape::Formatters::ActiveModelSerializers
+    helpers Grape::Helpers::ActiveModelSerializers
+  end
+end

lib/grape/formatters/active_model_serializers.rb

@@ -0,0 +1,15 @@
+# A Grape response formatter that can be used as 'formatter :json, Grape::Formatters::ActiveModelSerializers'
+#
+# Serializer options can be passed as a hash from your Grape endpoint using env[:active_model_serializer_options],
+# or better yet user the render helper in Grape::Helpers::ActiveModelSerializers
+module Grape
+  module Formatters
+    module ActiveModelSerializers
+      def self.call(resource, env)
+        serializer_options = {}
+        serializer_options.merge!(env[:active_model_serializer_options]) if env[:active_model_serializer_options]
+        ActiveModel::SerializableResource.new(resource, serializer_options).to_json
+      end
+    end
+  end
+end

lib/grape/helpers/active_model_serializers.rb

@@ -0,0 +1,16 @@
+# Helpers can be included in your Grape endpoint as: helpers Grape::Helpers::ActiveModelSerializers
+module Grape
+  module Helpers
+    module ActiveModelSerializers
+      # A convenience method for passing ActiveModelSerializers serializer options
+      #
+      # Example: To include relationships in the response: render(post, include: ['comments'])
+      #
+      # Example: To include pagination meta data: render(posts, meta: { page: posts.page, total_pages: posts.total_pages })
+      def render(resource, active_model_serializer_options = {})
+        env[:active_model_serializer_options] = active_model_serializer_options
+        resource
+      end
+    end
+  end
+end