Clarify that download rate is in bytes per second

I found https://github.com/rg3/youtube-dl/commit/f918ec7ea29a37521d1fc22fb9f900283c5a2c49 but it is still not clear to anyone who hasn't read Issue #723 whether the limit is in bits or bytes.  This is doubly confusing because 1) ISPs usually advertise speeds in bits per second, and 2) lowercase "k" and "m" are often used in correlation with bits rather than bytes.
Added exceptions for the subtitle and video types in .gitignore
Fixed small type issue
[dailymotion] Added support for subtitles + new InfoExtractor for
generic subtitle download.

The idea is that all subtitle downloaders must descend from SubtitlesIE
and implement only three basic methods to achieve the complete subtitle
download functionality. This will allow to reduce the code in YoutubeIE
once it is rewritten.
Undo the previous commit (it was a mistake)
[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.
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.
[subtitles] Added tests to check correct behavior when no subtitles are
available
Merge branch 'master' into subtitles_rework
Merge branch 'master' into subtitles_rework
Merge branch 'master' into subtitles_rework
Fixed issue #1277 KeyError when no description.

Allows a continue with a warning when an extractor cannot retrieve a description.
[daum] Get the video page from a canonical url to extract the full id (fixes #1373) and extract description.
Add extractor for tvcast.naver.com (closes #1331)
[generic] Recognize html5 video in the format '<video src=".+?"' and only unquote the url when extracting the id (fixes #1372)
[youtube] Playlists: extract the videos id from ['media$group']['yt$videoid'] (fixes #1374)

'media$player' is not defined for private videos.