Add chat sending support.

osu.Game/Online/API/Requests/PostMessageRequest.cs

@@ -0,0 +1,33 @@
+// Copyright (c) 2007-2017 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using osu.Framework.Extensions;
+using osu.Framework.IO.Network;
+using osu.Game.Online.Chat;
+
+namespace osu.Game.Online.API.Requests
+{
+    public class PostMessageRequest : APIRequest<Message>
+    {
+        private readonly Message message;
+
+        public PostMessageRequest(Message message)
+        {
+            this.message = message;
+        }
+
+        protected override WebRequest CreateWebRequest()
+        {
+            var req = base.CreateWebRequest();
+
+            req.Method = HttpMethod.POST;
+            req.AddParameter(@"target_type", message.TargetType.GetDescription());
+            req.AddParameter(@"target_id", message.TargetId.ToString());
+            req.AddParameter(@"message", message.Content);
+
+            return req;
+        }
+
+        protected override string Target => @"chat/messages";
+    }
+}