Make `IRCb` a `Duplex` stream

Avoid having to deal with creating and error handling on a connection,
and allow to use IRC protocol over streams other than TLS or TCP.

Do not require a callback to functions which don't return any actual
result.

Modify the example the show new API.

Modify the README accordingly.
Merge pull request #16 from mmalecki/stream

Make `IRCb` a `Duplex` stream