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,
It is necessary because of this:
```ruby
request_context[:uri].scheme = @primary_uri.scheme
request_context[:uri].host = @primary_uri.host
request_context[:uri].port = @primary_uri.port
```