Initial hack at supporting Notifications API

Octokit/Clients/NotificationsClient.cs

@@ -0,0 +1,18 @@
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+
+namespace Octokit
+{
+    public class NotificationsClient : ApiClient, INotificationsClient
+    {
+        public NotificationsClient(IApiConnection client) : base(client)
+        {
+        }
+
+        public async Task<IReadOnlyCollection<Notification>> ListNotifications()
+        {
+            return await Client.GetAll<Notification>(new Uri("/notifications", UriKind.Relative));
+        }
+    }
+}