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,
Right, that's unintended behavior in this case. Anyway we only need to reset the primary url in 2 casese: initialize and request failure, so I don't think we should modify it to call `first_or_next_solr_uri` on each request. My opinion is that the static @primary_uri is better and causes less overhead, even though not too elegant ;)