VS 2013 support (Windows SDK 8 and 8.1) plus wild VS targeting

This commit should address issue GH-150 and GH-185 (sorry for dragging my feet on this one)

The wild VS targeting feature works as follows.

- by default the VsVersions table contains all currently support versions
- it uses a `xpcall` to try and create the most recent available VS version installed
- `VsVersions` is an option that can be set to override this default behavior

A wild VS configuration is really wild, it can change based on external changes to the system and that will cause really weird build errors. You can always restrict the wild search pattern to avoid this, to some extent, but this is what you get with the wild option.

Let me know if you have any feedback or concerns about this patch. 

I've tested everything with Visual Studio 2008 Professional, Visual Studio 2010 Professional, Visual Studio 2012 Premium, Visual Studio 2012 Express, Visual Studio 2013 Preview and it seems to work just fine