Fixed #1503 -- Improved model validator to throw an error if a model doesn't...

Fixed #1503 -- Improved model validator to throw an error if a model doesn't manually define a primary key and has a field named 'id', which conflicts with the auto-primary-key convention. Thanks, mir@noris.de

git-svn-id: http://code.djangoproject.com/svn/django/trunk@3100 bcc190cf-cafb-0310-a4f2-bffc1f526a37