Fixed #251 -- Added OR support to queries, via the new 'complex' DB API keyword argument. Updated docs and added unit tests. Also removed old, undocumented '_or' parameter. Thanks, Hugo.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@1508 bcc190cf-cafb-0310-a4f2-bffc1f526a37