Implement the initial ratings application feature set

The first version (1.0) of the ratings application will allow users to rate a wiki page, and modify their rating a posteriori. An average rating for pages will then be available.
See for example the public site http://www.france2025.fr for an example of such application in XWiki.

See also the design page here : http://dev.xwiki.org/xwiki/bin/view/Design/RatingSystem