Support asynchronous call to prevent the app frozen in <code>initPoll()</code>.
According to MDN (https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest#open()),
<code>async: false</code> was deprecated at least in the latest version of Firefox.

> Starting with Gecko 30.0 (Firefox 30.0 / Thunderbird 30.0 / SeaMonkey 2.27),
> synchronous requests on the main thread have been deprecated due to the negative effects to the user experience.

And <code>async: false</code> will freeze the app for a notable while, so prevent using it.

The solution is using jQuery Deferred Object (http://api.jquery.com/category/deferred-object/),
the <code>done</code> method do the trick. (http://api.jquery.com/deferred.done/)

The <code>then</code> method is even better (http://api.jquery.com/deferred.then/),
however, it only supports jQuery 1.8 or later, which is high than PollJoy minimum requirement (1.6).