xpath_array with namespaces support

As a result of discussion with Peter, I provide modified patch for
xpath_array() with namespaces support.

The signature is:
  _xml xpath_array(text xpathQuery, xml xmlValue[, _text namespacesBindings])

The third argument is 2-dimensional array defining bindings for
namespaces. Simple examples:

xmltest=# SELECT xpath_array('//text()', '<local:data
xmlns:local="http://127.0.0.1"><local:piece id="1">number
one</local:piece><local:piece id="2" /></local:data>');
  xpath_array
----------------
 {"number one"}
(1 row)

xmltest=# SELECT xpath_array('//loc:piece/@id', '<local:data
xmlns:local="http://127.0.0.1"><local:piece id="1">number
one</local:piece><local:piece id="2" /></local:data>',
ARRAY[ARRAY['loc'], ARRAY['http://127.0.0.1']]);
 xpath_array
-------------
 {1,2}
(1 row)

Thoughts regarding other XPath functions were exposed a couple of days
ago: http://archives.postgresql.org/pgsql-patches/2007-02/msg00373.php

If there is no objections, we could call the function provided in this
patch as xpath() or xmlpath() (the latter is similar to SQL/XML
functions).

Also, maybe someone can suggest better approach for passing namespace
bindings (more convenient than ARRAY[ARRAY[...], ARRAY[...]])?

--
Best regards,
Nikolay