Fix multiple issues on render_multi_url example

1) The example runs multiple page instances concurrently and terminated
after the last URL is rendered but the previous renders do not always
completed since they are running asynchonously. This fix so that it
terminates only after all jobs are done.

2) render() was run immediately after page callback which generates
empty screen. This fix changed it to run only after 200 ms.

Issue: http://code.google.com/p/phantomjs/issues/detail?id=1021