Added convenience methods for to/from xml and json
diff --git a/src/ProtocolBuffers/ExtendableMessage.cs b/src/ProtocolBuffers/ExtendableMessage.cs
index b5fbac3..e8f9ce4 100644
--- a/src/ProtocolBuffers/ExtendableMessage.cs
+++ b/src/ProtocolBuffers/ExtendableMessage.cs
@@ -43,7 +43,7 @@
 {

     public abstract class ExtendableMessage<TMessage, TBuilder> : GeneratedMessage<TMessage, TBuilder>

         where TMessage : GeneratedMessage<TMessage, TBuilder>

-        where TBuilder : GeneratedBuilder<TMessage, TBuilder>

+        where TBuilder : GeneratedBuilder<TMessage, TBuilder>, new()

     {

         protected ExtendableMessage()

         {