be acknowledged by users with :
- portal_acknowledgement : tool that provided methods to retrieve
  document to acknowledge and to acknowledge them
- new document class : Acknowledgement
- possibility to have proxy of Documents. Acknowledgements will only
  be a proxy to event, the content of the message will not be kept
  in the event, and we can access it from the Acknowledgement thanks
  to the proxy
- the possibility to display site message on the top of the page
  with xhtml style
- an unit test


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@27216 20353a03-c40f-0410-a6d1-a30d3c3de9de