thumbnail_url accepts an optional size paramter

Valid sizes are: :default, :medium and :high
New Youtube playlists: find, create, add videos

* Replaced `account.perform!` with `account.like!`, `account.subscribe_to!`
* Added `account.add_to!` to add a video to an account’s playlist
* Added `account.find_or_create_playlist_by` to find or create an account’s playlist