Renames mongo id in sockets from "_id" to "mid"
Adds initial code for inviting users to rooms.
Moves chat input focus to correct location.
Implements user_list on serverside and clientside. The list is updated when users join and when users leave.
Replaces broken references to socket.handshake.session.mid with socket.mid.
Removes server socket function get_clients_in_room.function get_clients_in_room.