find video parameters in iframe

Some Tumblr sites seem to adopt a skin that puts the page with the video player and its parameters into a separate iframe.

This code kicks in if the "old way" fails.

The parameter format in this iframe is slightly different from the previous one.

Please note that the `default=""` in the new line 64 will suppress a previously present warning from `._search_regex` if no thumbnails are found.

I also added a test for a site currently using the iframe methode.
`video_thumbnails' is initialized in line 53.

And these values aren't guesses - they are extracted from the parameter list.