Add a helper to index monolingual objects

[sbaechler]

This backend assumes that every indexed model is multilingual. So for a given object all index entries share the same django-id.

This doesn't work for apps where the language is filtered on the manager level (i.e. the model has a language attribute).

Unfortunately the environment language is not set when haystacks index_queryset is evaluated.

A possible solution for this is a decorator class that wraps the queryset returned by index_queryset and filters it as soon as the language is available.

Haystack does slice the queryset before the language filtering can happen.