Added convenience methods for to/from xml and json
diff --git a/src/ProtocolBuffers/ExtendableBuilder.cs b/src/ProtocolBuffers/ExtendableBuilder.cs
index cbe203d..137762a 100644
--- a/src/ProtocolBuffers/ExtendableBuilder.cs
+++ b/src/ProtocolBuffers/ExtendableBuilder.cs
@@ -42,7 +42,7 @@
 {

     public abstract class ExtendableBuilder<TMessage, TBuilder> : GeneratedBuilder<TMessage, TBuilder>

         where TMessage : ExtendableMessage<TMessage, TBuilder>

-        where TBuilder : GeneratedBuilder<TMessage, TBuilder>

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

     {

         protected ExtendableBuilder()

         {