- add conversion modules for other Unicode encodings than UTF-8 
  (UTF-16, UCS-*)

- Better support for Windows, the locale detection stuff does not
  really work there.  Any help would be appreciated.

- Check whether other locale categories than LC_MESSAGES work.

- Cache all LC_* constants in Locale::gettext_pp

- Cache recoder objects in Locale::gettext_pp

- Only instantiate a Locale::Recode object if it is really necessary
  (and turn off the UTF-8 flag manually in that case)

- Avoid call to __load_domain in Locale::gettext_pp, use a cache table
  first
