Allow custom handling of property in json that doesn't exist in object #43

Given the following Json:

{
   ID: 1,
   OldProp: "value"
}

and the following class:
public class Foo {
   public int ID { get; set; }
   public string NewProp { get; set; }
}

Give the user a way of handling the fact that "OldProp" does not exist on 
the object.  The default will be to throw an exception.  The user can 
register a custom delegate to handle missing properties.  Typical actions 
could be to ignore the missing property or assign it to a different 
property in the case of a rename refactoring.