Allow waiting for async JS to capture the image

With this change, webkit2png is able to wait for arbitrary asynchronous
JavaScript to finish before capturing the screenshot. Use code such as:

    webkit2png.stop();  // wait for start() before capturing
    $.get("/path/to/handler", function(data) {
        displayData(data);
        webkit2png.start();
    });

and the picture won't be captured until the AJAX request returns. (I
stole the names 'start' and 'stop' from QUnit:
http://api.qunitjs.com/category/async-control/.)
Merge pull request #39 from spicyj/async-js

Allow waiting for async JS to capture the image