Set the "_play_proxy" boolean attribute to the video object
to pipe the video to mplayer.

Fix Youtube module test.

Signed-off-by: Vincent Texier <vit@free.fr>

The bug was solved by changing the maximum value of the 'search index'. Actually Youtube doesn't allow to get more than 500 results. That's why we changed the value of YOUTUBE_MAX_START_INDEX from 1000 to 500.

There is now a "latest" collection (or "latest_nsfw").
The feature didn't look much used, since it didn't work on many
backends.
Using collections will make it easy to support other things
like most viewed, featured, etc.
As a bonus, I added tests for every backend with the feature.

The regular expression is also more strict, yet accepts the
youtu.be short URLs and user page URLs.