Implemented publisher confirms

A channel can be put into publish confirm mode by calling
$channel->confirm_select();
If necessary a call back for ack'ed and nack'ed messages can be set calling
$channel->set_ack_handler($ackCallback);
or
$channel->set_nack_handler($nackCallback);
respectively.

For the callbacks to be called at all
$channel->wait()
must be called. I also implemented
$channel->wait_for_pending_acks()
Which calls wait() with the correct arguments but returns immediately in case there arent any pending acks.
It also provides a timeout parameter... just in case ;-)