Add new Avatar plugin category (with Gravatar and Libravatar)

This merge request following my ticket #333. It adds a new Avatar plugin category and two new plugins:
 - avatar.gravatar: Get user avatar from Gravatar
 - avatar.libravatar: Get user avatar from Libravatar

Avatar plugins are only called if the user doesn't have uploaded its own avatar (which is convenient in code and is also the more logical thing to do I think).