Fixed #342 -- added readonly_fields to ModelAdmin. Thanks Alex Gaynor for bootstrapping the patch.

      ModelAdmin has been given a readonly_fields that allow field and calculated
values to be displayed alongside editable fields. This works on model
add/change pages and inlines.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@11965 bcc190cf-cafb-0310-a4f2-bffc1f526a37