SDL client/server: the server can now manage clients disconnection
SDL client/server: add doxygen documentation
SDL client/server: add a handle_client_msg() function ; clients can now shutdown the server or make it print who is connected (shutdown and who commands)
SDL client/server: several improvements, add a quit command, and a SendMessage() function
SDL client/server: several code and design improvements, add a setup_client() function
SDL client/server: several improvements, add a handle_server_msg() function
SDL client/server: add a messages.h file for messages and commands macros