Upgraded Protobuf, and added api support for it

MediaBrowser.Model/Entities/UserItemData.cs

@@ -1,13 +1,18 @@
 using System;
+using ProtoBuf;
 
 namespace MediaBrowser.Model.Entities
 {
+    [ProtoContract]
     public class UserItemData
     {
+        [ProtoMember(1)]
         public UserItemRating Rating { get; set; }
 
+        [ProtoMember(2)]
         public long PlaybackPositionTicks { get; set; }
 
+        [ProtoMember(3)]
         public int PlayCount { get; set; }
     }