Case-insensitive lookup by default

I wanted lookups on tags to be case insensitive by default, so that things like Tag.objects.get(name='Tag') would return any similar tags (ignoring case differences), i.e. `<Tag: tag>`. This snippet makes lookup on the 'name' field case-insensitive by default, although case-sensitive lookups can still be achieved with 'name__exact'. Methods like get_or_create will work as expected and be case-insensitive.