[teachertube] Fix extraction for Python 3
[dreisat] Fix thumbnails' width and height
[dreisat] Modernize
[veoh] Capture error message
[nuvid] Improve extraction
[nuvid] Remove superfluous slash
rtmp and hls downloaders: Clarify error message when the external tools are not installed

Ask to install them, as we do in the postprocessor.
We get some reports with it, like #3061 or #3048.
[generic] Add support for <embed YouTube
[hypem] Modernize
[Vulture] Add support for vulture.com
Remove unused import
release 2014.06.09
[ndtv] Fix title extraction and modernize
[tube8] Update test and modernize
[blinkx] Modernize
[ted] Update test md5
[livestream] Modernize
[rai] Add support for Rai websites (Closes #2930)