Extend snippet to include all the YouTube methods

Now resources (Channel, Video, Playlist, PlaylistItem) have access
to all the data made available by YouTube in the 'snippet' part.
All the following methods are now available:

video.channel_id #=> "UCxO1tY8h1AhOz0T4ENwmpow"
video.channel_title #=> "Fullscreen"
video.category_id #=> "22"
video.live_broadcast_content #=> "none"
playlist.tags #=> []
playlist.channel_id #=> "UCxO1tY8h1AhOz0T4ENwmpow"
playlist.channel_title #=> "Fullscreen"
item.channel_id #=> "UCxO1tY8h1AhOz0T4ENwmpow"
item.channel_title #=> "Fullscreen"
item.playlist_id #=> "PLSWYkYzOrPMRCK6j0UgryI8E0NHhoVdRc"
item.video_id #=> "W4GhTprSsOY"

As part of this commit, PlaylistItem now inherit from Resource, and
therefore also has access to the status methods:

item.public? #=> true
Merge branch 'extend-snippet'