- don't get the value from the field, this should be done by the caller and
  passed to methods using value parameter (in render_*, value should be "ready
  to use")
- change the way to get attributes because the dict structure change
- add a renderer for DateTime fields

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@31495 20353a03-c40f-0410-a6d1-a30d3c3de9de