Various improvements and cleanups, most notably:

• Fixed Client proxy requests
  
• Added client verification when a verify file is passed to Server
  
• Added session_id_context for session reuse
  
• Server errors are now passed to an on_error std::function if set
  
• Added crypto.hpp for convenient C++ wrappings for commonly used OpenSSL functions
  
• Cleanup of server-constructors. Previous constructors have been marked as deprecated
  
• Added on_upgrade std::function for cases where one wants to handle connection upgrades by another library, for instanace Simple-WebSocket-Server
  
• Timeout can now be set for Client requests
  
• Client now supports Connection: close
  
• Added support for request header Connection: keep-alive in Server
  
• Added Server::Request::parse_query_string()