IDeserializationCallback & .NET 3.5 support

Hello,

Thanks for this very good serializer. I require to have the following small modifications in order to use it on other projects.

Modifications are:
* support for .NET 3.5: Enum.GetUnderlyingType and ILGenerator.ThrowException
* support for IDeserializationCallback (see GenDeserializerBody)

Cheers,
- Pierre