refactored names, restructured and added structs (now have worker_t, user_t, and chat_room_t types).
added chat_server.h header file. no new functionality in this commit.
added read_client() wrapper function to simplify interface for reading from client.
added functionality to obtain and use a username instead of user id to identify clients during chat, entry, and exit.
Merge branch 'master' of github.com:jeremygreenwood/CST340-chat