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
XPath: Rename xml_node::select_single_node to ::select_node

select_node is shorter and mistyping nodes as node or vice versa should
not lead to any issues since return types are substantially different.

select_single_node method still works and will be deprecated with an
attribute and removed at some point.

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1065 99668b35-9821-0410-8761-19e4c4f06640
docs: Update XPath documentation

Add documentation for xpath_query::evaluate_node and change
select_single_node to select_node in documentation and samples.

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1066 99668b35-9821-0410-8761-19e4c4f06640