Some warning fixes, and UTF-8 support on Windows.

My fork contains a few patches that you may be interested in. One adds the LUAMOD_API macro introduced in Lua 5.2. The next few fix some warnings when compiling with MSVC. Finally the last one, and most important, adds support for UTF-8 paths if the Lua library has been compiled with my wstring patch (which exports the LUA_HAS_WSTRING define for detection).