http connection string hardcoded

[jorgjanke]

In ElasticsearchReporter.openConnection(..)
the protocol is hardcoded

URL templateUrl = new URL("http://" + host  + uri);

(line 439) - just noticed also reported by #17

Please add the ability to modify the protocol in the builder
or add logic like

String theHost = host;
if (!host.contains("://") {
  theHost = "http://" + host;
}
URL templateUrl = new URL(theHost  + uri);