`multi_return<>` name to `return_type<>` .
Makes more sense, as the return does not always have to be multiple types.
(Indeed, it is specialized for 1 and 0 cases).
We now have the ability to get a userdata that has been created C++ style out of lua
Using `auto` and `decltype` in more places that MSVC can handle it -- using type traits in other places to avoid VC++'s chokes
More flexibility, woo!
Merge pull request #19 from ThePhD/master

Getting userdata out of lua into C++ (after coming from C++ originally)