Add ribbon/zuul features and docs plus a sample

spring-cloud-sleuth-samples/spring-cloud-sleuth-sample-messaging/src/main/java/sample/SampleMessagingApplication.java

@@ -72,7 +72,7 @@ public class SampleMessagingApplication {
 
 	// Use this for debugging (or if there is no Zipkin collector running on port 9410)
 	@Bean
-	@ConditionalOnProperty("span.logging.enabled")
+	@ConditionalOnProperty(value="sample.zipkin.enabled", havingValue="false")
 	public SpanCollector spanCollector() {
 		return new LoggingSpanCollectorImpl();
 	}