Improved syndication feed framework to use RequestSite if the sites framework is not installed -- i.e., the sites framework is no longer required to use the syndication feed framework. This is backwards incompatible if anybody has subclassed Feed and overridden __init__(), because the second parameter is now expected to be an HttpRequest object instead of request.path

git-svn-id: http://code.djangoproject.com/svn/django/trunk@5654 bcc190cf-cafb-0310-a4f2-bffc1f526a37