Check that client is playing the right item

Send date when playback command is emitted
Rename some classes

MediaBrowser.Model/Syncplay/JoinGroupRequest.cs

@@ -0,0 +1,22 @@
+using System;
+
+namespace MediaBrowser.Model.Syncplay
+{
+    /// <summary>
+    /// Class JoinGroupRequest.
+    /// </summary>
+    public class JoinGroupRequest
+    {
+        /// <summary>
+        /// Gets or sets the Group id.
+        /// </summary>
+        /// <value>The Group id to join.</value>
+        public Guid GroupId { get; set; }
+
+        /// <summary>
+        /// Gets or sets the playing item id.
+        /// </summary>
+        /// <value>The client's currently playing item id.</value>
+        public Guid PlayingItemId { get; set; }
+    }
+}