Version 0.8.0 - CMake Updates

• [Breaking] Library initializer now uses a static singleton created via socket_initializer::initialize() call, which can be called repeatedly with no ill effect. Also added global socketpp::initialize() function as shortcut.
• Improvements to CMake to better follow modern standards.
  • CMake required version bumped up to 3.12
  • Generating CMake files for downstream projects (config, target, version)
  • Windows builds default to shared DLL, not static library
  • Lots of cleanup