Replace use of BEGIN_NS/BEGIN_\w+_NS with namespace \w+

Was a bad idea.
Remove use of static, use anonymous namespace.
Add S_EXPORT for exported symbols.

Redefined in log.cc because it's a special case (intended to have zero
dependencies inside snow-common).
Add dylib building junk.
Add sha256.cc/hh. Also openssl dependency.

Currently uses a path hardcoded for a newer version of OpenSSL than
what's available on Mac OS X. Need to find some way to get tup to import
stuff from pkg-config..