add access_control and comment_count to video entries

[rainchen]

I need the "comment_cout", +1 for this pull. I preferred name it as comments_count, but considering there is a view_count and favorite_count, the "comment_count" would be OK.

And how about adding a shortcut method for getting comments for a video, e.g.:

video.comments # equals youtube_client.comments(video.unique_id)

[yaauie]

The architecture of youtube_it does not lend itself well to chaining like that, and frankly things are enough of a mess that to enable this in a maintainable way would require a very major overhaul or complete rewrite of the entire gem, in the process breaking all sorts of expectations the existing version creates.