Added avatar's CSS markup to differentiate between regular commenters and moderator

I thought it would be more convenient to add a CSS class to the moderator's avatar so webmasters could easily change its background color as it is not something we can do via the dashboard.

As a side note: Maybe would it be better to add the class "commento-moderator" to the comment's parent div instead of adding it to each HTML node?

Hope you find it useful!