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