lua + pugixml via LuaBridge

Started working on a binding of pugixml for lua:

pugixml supports utf-8, so, using icu4lua one could, in principle, have a nice lua-style processing of XML files. The interface is right now kept almost one-to-one to the original. It shows, at least for me, how easy it is to make bindings to C++ using LuaBridge and how easy the usage of pugixml is.