refactor header define guards

This change was done to follow Boost guidelines:
http://www.boost.org/development/header.html
refactor: embedded_server_socket.cpp -> socket.cpp
add server_socket_adaptor<std::reference_wrapper<T>> specialization
remove server_socket_reference_adaptor

It's intended use case is now handled properly by
polymorphic_server_socket<std::reference_wrapper<T>>