Retrieve YouTube content and metadata

Features:

 - Retreive metadata such as viewcount, duration, rating, author, thumbnail, keywords
 - Download video or audio at requested resolution / bitrate / format / filesize
 - Command line tool (ytdl) for downloading directly from the command line
 - Retrieve the URL to stream the video in a player such as vlc or mplayer
 - Works with age-restricted videos and non-embeddable videos
 - Small, standalone, single importable module file (pafy.py)
 - Select highest quality stream for download or streaming
 - Download video only (no audio) in m4v or webm format
 - Download audio only (no video) in ogg or m4a format
 - Retreive playlists and playlist metadata
 - Works with Python 2.6+ and 3.3+
 - Optionally depends on youtube-dl (recommended; more stable)
