Add basic online API support.

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

@@ -0,0 +1,37 @@
+//Copyright (c) 2007-2016 ppy Pty Ltd <contact@ppy.sh>.
+//Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using System.Collections.Generic;
+using osu.Framework.IO.Network;
+using osu.Game.Online.Social;
+
+namespace osu.Game.Online.API.Requests
+{
+    internal class GetMessagesRequest : APIRequest<List<Message>>
+    {
+        List<Channel> channels;
+        long? since;
+
+        public GetMessagesRequest(List<Channel> channels, long? sinceId)
+        {
+            this.channels = channels;
+            this.since = sinceId;
+        }
+
+        protected override WebRequest CreateWebRequest()
+        {
+            string channelString = string.Empty;
+            foreach (Channel c in channels)
+                channelString += c.Id + ",";
+            channelString = channelString.TrimEnd(',');
+
+            var req = base.CreateWebRequest();
+            req.AddParameter(@"channels", channelString);
+            if (since.HasValue) req.AddParameter(@"since", since.Value.ToString());
+
+            return req;
+        }
+
+        protected override string Target => @"chat/messages";
+    }
+}