PKG: Edits to normative references

I've just completed a pass through the UBL 1.0 normative
references and made the following changes:

 - [CCTS] reference updated to 2.01

 - [ISO 8601] removed from list because not referenced anywhere in
   the text

 - [ISO11179] now points specifically to Part 1 (containing the
   definitions used in UBL)

 - [RNG] has been added in anticipation of an informative appendix
   containing RNG versions of the UBL schemas

 - All normative references have been copied to the OASIS site and
   their URLs added as alternatives to the official ones so that
   the versions normative to 1.0 remain available for future
   reference

Jon