Posts external comments count

Based on some discussion with @joelturnbull and @rockwood I think we may need to change the /authors resource to something more comment oriented.

Eventually there will be calls specifying date ranges and tying that to authors does not make sense.

/comments.json?start_date=2013-07-01
/comments.json

When we do move to /comments.json instead of /authors.json we'll need to update rack-cors allow list in dashboard/config/application.rb https://github.com/gaslight/dashboard/commit/7262a67a8523a49c6b6364d13289e0f1b1d56636

/cc @cdmwebs