Add YoutubePlaylist

Hi again,

I have created a wrapper around YouTube Playlists.

Its supposed to work exclusively for YouTube playlists formatted like http://www.youtube.com/playlist?p=PLA575C81A1FBC04CF because http://www.youtu.be/playlist?p=PLA575C81A1FBC04CF doesnt appear to exist. As far as I know this is only the only want to directly access a playlist on YouTube. 

I know you can also have http://www.youtube.com/watch?v=4Y8M0RPvBWo&list=PLA575C81A1FBC04CF&index=26 but I think this is best served being wrapped up by Youtube the class and not the new YoutubePlaylist class.

As I see it

youtube.com/watch or youtu.be == Youtube
youtube.com/playlist == YoutubePlaylist

Let me know what you think :+1: 

Thanks
Karl