Add hook for custom auto-index action

At Causes.com, we want to index into solr asynchronously.  We like the tooling around auto_index, but wanted the call to solr_index to be done asynchronously.

If we could pass in a callable to use instead of the default solr_index, it would decouple the need to index from exactly how indexing is done.

We then are able to extend Sunspot to do the async indexing.

(This is a patch against 1.3, which I appreciate is now old - just offering upstream in case it's useful.)