Bump version to 0.4.

Parameter API changes.

* "defined?"     becomes "set?".

* "when-defined" becomes "when-complete".

* The "default" value is now handled as int.rep, not going through
  validation. (Like the result of the "generate" callback).

* Updated validation type "default" to always provide something.

* Standard VT fail now takes parameter instance command as first
  argument (see also below about callbacks), and uses it to generate
  an enhanced error message (naming the parameter at issue).

Callback API changes.

* All callbacks now take the parameter instance command as their first
  argument.

* Removed the (class)methods to dynamically locate and access a
  innermost config instance on the stack.

* As replacement added method "config" to make the container
  accessible from each parameter it contains, and thus all other
  parameters as well.

* Update all validation types.