Hack, hack, hack ... Mostly adding functions:

- FEATURES TODO README INSTALL: updated
- libxslt/xslt.h: added URL and version/vendor :-)
- libxslt/transform.c: fixed a problem in xsl:attribute, removed
  attempt to support older libxml2 version.
- libxslt/variables.h libxslt/xsltInternals.h: update to structures
  and macros to add/register new document created by document()
- libxslt/functions.c: implemented current(), unparsed-entity-uri()
  system-property(), element-available() and function-available().
  A crippled version of document() has been added too.
Daniel