Rename DetailsSet to ContentDetail
Add StatisticsSet to Video, Channel

New methods available are:

- channel.view_count #=> 421619
- channel.comment_count #=> 773
- channel.video_count #=> 13
- channel.subscriber_count #=> 136925
- channel.subscriber_count_visible? #=> true
- video.view_count #=> 55843
- video.comment_count #=> 308
- video.like_count #=> 556
- video.dislike_count #=> 78
- video.favorite_count #=> 0
Merge branch 'add-statistics'