.unknown_video -> anon-v.com site support (Anon-IB network)

[ghost]

please add support for anon-v

> youtube-dl -v http://anon-v.com/videos/2453/almost-caught/
[debug] System config: [u'--restrict-filenames', u'--ignore-errors', u'-n', u'-o', u'%(title)s-%(resolution)s.%(ext)s', u'-f', u'bestvideo[ext!=webm]+bestaudio[ext!=webm]/best[ext!=webm]']
[debug] User config: []
[debug] Command-line args: [u'-v', u'http://anon-v.com/videos/2453/almost-caught/']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2015.11.19
[debug] Python version 2.7.10 - Darwin-15.0.0-x86_64-i386-64bit
[debug] exe versions: avconv 11.4, avprobe 11.4, ffmpeg 2.8.2, ffprobe 2.8.2, rtmpdump 2.4
[debug] Proxy map: {'http': 'http://localhost:8118', 'https': 'http://localhost:8118'}
[generic] almost-caught: Requesting header
WARNING: Falling back on generic information extractor.
[generic] almost-caught: Downloading webpage
[generic] almost-caught: Extracting information
[debug] Invoking downloader on u'http://anon-v.com/get_file/1/47ce6814081c19fdcb08c7153f5c8322/2000/2453/2453.mp4/'
[download] Destination: ALMOST_CAUGHT_Anon-V.com-NA.unknown_video
[download] 100% of 94.65MiB in 46:58

the downloaded file doesn't have extension

-rw-r--r--   1 ceremy  staff  99248266 Apr  4  2015 ALMOST_CAUGHT_Anon-V.com-NA.unknown_video

> file ALMOST_CAUGHT_Anon-V.com-NA.unknown_video 
ALMOST_CAUGHT_Anon-V.com-NA.unknown_video: ISO Media, MPEG v4 system, version 1