PollWatcher: unify signature of new and with_delay

[jhscheer]

• Move event_handler logic from new to with_delay to be able to call:

let (tx, rx) = std::sync::mpsc::channel();
let mut watcher = PollWatcher::with_delay(tx, Duration::from_secs(1))?;

instead of:

let (tx, rx) = std::sync::mpsc::channel();
let mut watcher = PollWatcher::with_delay(Arc::new(Mutex::new(tx)), Duration::from_secs(1))?;

• Add documentation

[0xpr03]

Thanks, I thought a bit about whether we may want to allow passing an Arc inside, but ultimately we already do the same thing for other backends..