Manage gravatar for user thumbnails

Would be nice to include the Gravatar macro:
http://extensions.xwiki.org/xwiki/bin/view/Extension/Gravatar+Macro

If a wiki profile is found for an email, and an avatar is found in the wiki profile, it is used in priority.
If not, we add it with gravatar macro, with some default picture (like monsterid).

Would be better to implement this as a macro to be used a bit everywhere in the app (livetable home page, timeline, topics views, ...)
It should be possible to deactivate the gravatar feature, for those users having no access to internet from their browser.