Use append_new_node in node_copy_tree

This bypasses the allow_insert check (which is redundant for copying since
we're mirroring an existing node structure that must be valid) and does
not cause an extra allocation for new declaration nodes. Overall results
in 15% faster copying,

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1036 99668b35-9821-0410-8761-19e4c4f06640