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
Enforce minimum version requirements on OpenSSL and Boost
Make is_set_bit a universal template function
Refactor usage of boost::format
Add beast::tagged_integer:
 * Wraps standard integer types to provide type-safety
 * Named types provide self-documenting semantics
 * Catches programmer errors involving mismatched types at compile time
 * Operators restrict mutation to only safe and meaningful operations
Add CleanCCPFiles.py, a utility for tidying up whitespace
Refactor ripple_overlay
  * Use rvalue move to receive accepted sockets
  * Split asio dependent APIs to their own class and file
  * Update documentation
  * Organize code into different files
  * Make some members private
  * Rename things for clarity