XPath: Introduce xpath_query::evaluate_node

This method is equivalent to xml_node::select_single_node. This makes
select_single_node faster in certain cases by avoiding an allocation and -
more importantly - paves the way for future step optimizations.

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1064 99668b35-9821-0410-8761-19e4c4f06640