New Zero struct implements "compare with zero."
* Zero lets classes efficiently compare with 0, so
  you can use constructors like x < zero or y != zero.
* New BEAST_CONSTEXPR to handle Windows/C++11 differences
  regarding the constexpr specifier.
Updated example cfg for new validator public keys.

Signed-off-by: The Doctor <drwho@virtadpt.net>
Fix Visual Studio 2013 build