[youtube] Fix download of subtitles with '--all-subs'

If _extract_subtitles is called the option 'write subtitles' is always true.
Add a post processor for embedding subtitles in mp4 videos (closes #1052)
XHamsterIE: Fix video extension

Cut off GET parameter
XHamsterIE: Add video description
Merge pull request #1301 from rzhxeo/xhamster

XHamsterIE: Fix video extension and add video description
[xhamster] use determine_ext
Print a message before embedding the subtitles
Allow to specify multiple subtitles languages separated by commas (closes #518)
[youtube] update algo for length 86 (fixes #1302)
release 2013.08.23
Switch to domain yt-dl.org