Add issue representation

lib/github/representation/issue.rb

+module Github
+  module Representation
+    class Issue < Representation::Base
+      def iid
+        raw['number']
+      end
+
+      def title
+        raw['title']
+      end
+
+      def description
+        raw['body'] || ''
+      end
+
+      def milestone
+        return unless raw['milestone'].present?
+
+        @milestone ||= Github::Representation::Milestone.new(raw['milestone'])
+      end
+
+      def author
+        @author ||= Github::Representation::User.new(raw['user'])
+      end
+
+      def assignee
+        return unless assigned?
+
+        @assignee ||= Github::Representation::User.new(raw['assignee'])
+      end
+
+      def state
+        raw['state'] == 'closed' ? 'closed' : 'opened'
+      end
+
+      def url
+        raw['url']
+      end
+
+      def created_at
+        raw['created_at']
+      end
+
+      def updated_at
+        raw['updated_at']
+      end
+
+      def assigned?
+        raw['assignee'].present?
+      end
+
+      def pull_request?
+        raw['pull_request'].present?
+      end
+    end
+  end
+end