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
The fact that you had to add this extra line makes me think that what we really want to instrument is the whole `run` method, and not just this single line.

Maybe we can leave the `run` method as it is, and have the instrumentation *around* the whole `run` method.

Does it make sense?