Add new approach to autocomplete_search_fields

I found myself needing auto-complete on a model I couldn't alter, and felt wrapping it in a Proxy model was too much work for just this one case.

So I've added AUTOCOMPLETE_SEARCH_FIELDS to settings, which is a two-layer dict of [app_label][model_name] to get a list of search fields.

This way, you can augment 3rd party apps easily and cleanly, albeit in a slightly more limited fashion.

Also, if the requested model does not have any solution, it will no longer explode.