Add HTTP Auth and SSL to the ES output plugin

The typical use case is the placement of a transparent proxy in front of
ES. This PR enables basic Auth and HTTPS to be configured independently.

While the SSL configurations only allow the validation of the server's
certificate and specification of the CA cert, it's easy to add the
necessary options to send the client's certificate.

Also, this patch removes the use of FTW in the output plugin, sticking only to the elasticsearch-ruby client. If performance is an issue, a faster alternative to the faraday transport in es-ruby could be implemented.
`:validate => :path` will validate that the path exists and provide users with decent feedback when it fails.