Rename messages-client to demo-client

Issue gh-1189

Author: jgrandja

samples/messages-client/gradle.properties renamed to samples/demo-client/gradle.properties

@@ -23,10 +23,10 @@
  * @since 0.0.1
  */
 @SpringBootApplication
-public class MessagesClientApplication {
+public class DemoClientApplication {
 
 	public static void main(String[] args) {
-		SpringApplication.run(MessagesClientApplication.class, args);
+		SpringApplication.run(DemoClientApplication.class, args);
 	}
 
 }