I don;t think it should be in the subsite namespace since parameters don't map
in any meaningful way to subsite. Also I am not sure whats easily available
in the parameter::set function but I would think we would want to provide more
than just those args (ie. if we have a parameter_id we should pass that as well).
Finally, the argument and callback docs should be in the callback decleration
in preference to the callback implementation.
