Removed use of deprecated lua_*unsigned().
Implemented Lua::ScopedRef.
Updated README.md to reflect recent changes.

- FMOD Studio is now the default backend on all platforms.
- Added information regarding Visual Studio 2013 (Express) Update 3.
Compile fixes and other oddities.

- Linux: Added missing #include.
- Linux: Fixed failing tests.
- Fixed some GCC warnings.
Implemented two more getters for Sprite.
Added 'zip' and 'zip_with' to Functional.lua.