taxonomy_xml 6.x-1.1

This is actually a feature release, on top of an unreleased first port... but seeing as the D6 port is brand new, I'm adding my changes (also seen in DRUPAL-5--2) here at the root of the branch. Also because I failed to be clever with CVS.

Anyway - this version
- works for me, supports the older format as good as it ever did (AFAIK)
- adds CSV and RDF import/export formats
- split the format handlers off into their own libraries.
- Has some documentation added

@see http://drupal.org/node/299599