Added half of oracle backend. (The other half is all of the special-casing in...

Added half of oracle backend. (The other half is all of the special-casing in django/db/models/query.py, which I will be refactoring.) Refs #1990. Thanks, Jason Huggins and tzellman

git-svn-id: http://code.djangoproject.com/svn/django/trunk@2986 bcc190cf-cafb-0310-a4f2-bffc1f526a37