Added CORS to Making Cross-Domain Requests

inspired By: https://github.com/chrisbu/dartlang_json_webservice_article_code/blob/master/simpleserver/simpleserver.dart

now you can enable or disable CORS with "cors" bool parameter
WebServer server = new WebServer(... ...  "other parameter" cors:true );

where are used: 

void _send_response(HttpResponse response, ContentType contentType, String result) {
 -    response
 +  	if(this.Cors==true){
 +  		response
 +        ..headers.add("Access-Control-Allow-Origin", "*, ")
 +        ..headers.add("Access-Control-Allow-Methods", "POST, GET, OPTIONS")
 +        ..headers.add("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
 +    }
... ... ...