Playlists (I give up, implemented on the API)

Simply switching to the public API killed three nasty bugs.

Then I corrected the "is it a video or a playlist" mismatch by making `YoutubeIE.suitable` first run `YoutubePlaylistIE.suitable` and return `False` on positive. Trust me, current behavior can't be reproduced with sane regexes.

(Plus some other PL-related changes and the definition as `@classmethod`s of `IE.working` and `IE.suitable`)