Better Gravatar generated icons

Description

Are you bored by Identicons and MonsterId icons in your blog comments?
This plugin will add new avatars for users that don’t have a Gravatar image setted, to use this choose the “blank” icon in your “Discussion settings” and use this plugin setting panel to choose your new avatar style. Avatars come from these 3rd party services: Flat Hash (from http://flathash.com/), Robot and monsters Hash (from https://robohash.org/) and Unicorns avatars (from https://unicornify.appspot.com/).