Added away(msg, optional callback) + ignored rpl_luserunknown + emit 'unhandled'

1) Added (with doc & examples) away functionality:

bot.away('Leave a message', function(serverResponse) {
    console.log('Server responds with..');
});

bot.away(); // remove away

2) Added rpl_luserunknown to ignored

3) Emit 'unhandled' event, just in case there's something missing (special) so you can handle it without changing the source code for node-irc.