Clone attributes as needed. Added Node.xpath, Node.match.

Clone attributes as needed. Added Node.xpath, Node.match.

 * Attributes are cloned when inserted in a tree if they're already part
   of another tree.

 * Added Node.xpath as canonical name to query a tree with a XPath
   expression (and with [] and / operator overloaded to call it.)

 * Added Node.match to test node against a XSLT pattern.
git-archimport-id: frederic@jolliton.com--2005-main/tx--main--0.1--patch-42