Bring tests from wpcom and refactor for Jetpack.

Took a shot at bringing over the tests from wpcom. There's no hulu support (afaik), so I modified the tests to reflect that. Also, there were some issues with the audio and wpvideo ids not being discovered by extract; I put a potential fix for that. Is moving the function out of videopress.php and into shortcodes a reasonable thing to do?
Tests referenced here originally: https://github.com/Automattic/jetpack/pull/1048