Add pull_requests attribute to Story

In order to get it populated, `fields: 'pull_requests'` is required when
calling the method that retrieves the story(ies) - otherwise it's nil

lib/tracker_api.rb

@@ -89,6 +89,7 @@ module Shared
     autoload :PrimaryResource, 'tracker_api/resources/primary_resource'
     autoload :Project, 'tracker_api/resources/project'
     autoload :ProjectMembership, 'tracker_api/resources/project_membership'
+    autoload :PullRequest, 'tracker_api/resources/pull_request'
     autoload :Workspace, 'tracker_api/resources/workspace'
     autoload :SearchResultContainer, 'tracker_api/resources/search_result_container'
     autoload :StoriesSearchResult, 'tracker_api/resources/stories_search_result'

lib/tracker_api/resources/pull_request.rb

@@ -0,0 +1,22 @@
+module TrackerApi
+  module Resources
+    class PullRequest
+      include Shared::Base
+
+      attribute :client
+
+      attribute :id, Integer
+      attribute :story_id, Integer
+      attribute :epic_id, Integer
+      attribute :owner, String
+      attribute :repo, String
+      attribute :number, Integer
+      attribute :host_url, String
+      attribute :original_url, String
+      attribute :status, String
+      attribute :created_at, DateTime
+      attribute :updated_at, DateTime
+      attribute :kind, String
+    end
+  end
+end

lib/tracker_api/resources/story.rb

@@ -30,6 +30,7 @@ class Story
       attribute :owners, [Person]
       attribute :planned_iteration_number, Integer
       attribute :project_id, Integer
+      attribute :pull_requests, [PullRequest]
       attribute :requested_by, Person
       attribute :requested_by_id, Integer
       attribute :reviews, [Review]