Add a polling method in the IO

[snowmantw]

For those polling required method, we should add a polling method like this:

IO().polling('/wait_something', 400, 3, check).done()

It means polling the server with endpoint '/wait_something' every 400ms, and automatically turns failed if it didn't get right answer, which is neither caused by errors (handle by the method implicitly) nor fails to pass the check more than 3 times.