Add Dummy backend

This adds a dummy backend. It can easily be switched to in django's local_settings.py by adding:

```
HAYSTACK_CONNECTIONS = {
     'default': {
         'ENGINE': 'haystack.backends.dummy_backend.DummySearchEngine',
     },
 }
```

Note that this file is slightly different than the dummy_backend.py file that it is derived from in the original haystack. It adds DummySearchEngine which doesn't exist.