Support private videos

Some videos on dailymotion are "private", which means they have a url that is only accessible if you know the URL.

For these videos, instead of :
http://www.dailymotion.com/video/{video id}_{clean url}
URLs are of type :
http://www.dailymotion.com/video/{video id (secret)}

Moreover, for these videos, thumbnails cannot be retrieved using :
http://www.dailymotion.com/thumbnail/video/{video id}
Instead we need to use the API :
https://api.dailymotion.com/video/{video id}?fields=thumbnail_url
And use the returned URL.

This patch addresses both issues.