Allow subsecond accuracy when reading from pubsub

In environments where rapid flow is key (for example in reactphp) it is crucial to be able to quickly poll for new messages in a pubsub channel without waiting for at least a second.
With three small changes in Channel.php this is possible by specifying the timeout as float. so for example a timeout of 0.2 is possible with this.
Unfortunatly there are no unit-tests yet for pubsub so I did not write one but I would be willing to write some tests if needed.