Add support for multiple nodes, so the :url option could be an array of URLs.

Allows to pass an array of URLs, and in case of `Errno::ECONNREFUSED` RSolr will now try another nodes (if available) before raising the error,
> first_or_next_solr_uri
It does not return that url, thus `select_primary_url` reflects exactly what it does, but you are right, this shouldn't be a public method.