[internal] Improved subtitle architecture + (update in
youtube/dailymotion)

The structure of subtitles was refined, you only need to implement one
method that returns a dictionnary of the available subtitles (lang, url) to
support all the subtitle options in a website. I updated the subtitle
downloaders for youtube/dailymotion to show how it works.
Clarify template error message (#1209)
Merge branch 'master' of github.com:rg3/youtube-dl
Separated subtitle options in their own group
[subtitles] removed only-sub option (--skip-download achieves the same
functionality)
[test] Cleaned subtitles tests
[subtitles] Improved docs + new class for servers who don't support
auto-caption
[subtitles] Skips now the subtitles that has already been downloaded.

Just a validation for file exists, I also removed a method that wasn't
been used because it was a copy paste from FileDownloader.
Invalid signature again (fixes #1215)
Merge pull request #1216 from patrickslin/patch-5

Invalid signature again (fixes #1215)
release 2013.08.08.1
[subtitles] Added tests to check correct behavior when no subtitles are
available
New sig len 89 algo

Fixes new YT encrypted sig len 89.
Merge pull request #1218 from patrickslin/patch-2

New sig len 89 algo
release 2013.08.09
add an aes implementation
 YoupornIE: Add support for hd videos and update Test
Note update possibility on errors (thanks @chbrown, #1229)
[vevo] fix testcase
Added an IE for hark.com