Implement recv_message/0 in terms of recv_message/1

Just give it the special timeout value of 'infinity'. (There's also
the option of giving recv_message/1 `0`, which means, return if
there's not a message there right now).
Add poll_message/0

Equivalent, aside from the failure return value, to `recv_message(0)`.

Mention it in the README, and fix up the examples so that the results
match what you get if you run them.