WinXP compatibility update

This changes the Platform Toolset configuration from "Visual Studio
2013 (v120)" to "Visual Studio 2013 - Windows XP (v120_xp)".  Without
this change, executables built by VS2013 will not run on WinXP.

To actually run on WinXP, we also need to install the redistributable
msvcr120.dll and mfc120u.dll, both of which are fairly large.  The
installation package has more than doubled in size.

At some point we may want to drop WinXP support -- Microsoft declared
end-of-life on April 8 2014 -- but if the only penalty is a 2MB increase
in installer size, we might as well keep supporting WinXP users.