Add front-end graphics, interactivity.

Add:
	src/chat_client.c
		-add client functionality: user can view a live stream of messages, and
		enter text into a buffer; input will be interpreted either as a message,
		or a command.

	src/constants.h
		-change PORT_NUMBER to 50001, because old port wasn't cleaning out
		quickly (after a dirty shutdown, chat_server couldn't immediately open
		a socket).
Final commit.
Add keyboard.