Add AS::Notifications to YouTube requests

Opening this PR for discussion.

ActiveSupport::Notifications, by convention, should be namespaced right-to-left, so I've named this `request.yt` (feel free to change this though).

Could be useful for:
  * Quota / Request analysis
  * Time spent making YouTube requests
  * Audit Trails
Look at what [ActiveResource does](https://github.com/rails/activeresource/blob/82eb29ab023b3105b29bce31c9a00a3b9a9653aa/lib/active_resource/connection.rb#L118L122).