Add Atmosphere sample application

Add Atmosphere example application based on
http://github.com/Atmosphere/atmosphere-samples/tree/master/samples/chat

Closes gh-2341

spring-boot-samples/pom.xml

@@ -27,6 +27,7 @@
 		<module>spring-boot-sample-actuator-ui</module>
 		<module>spring-boot-sample-amqp</module>
 		<module>spring-boot-sample-aop</module>
+		<module>spring-boot-sample-atmosphere</module>
 		<module>spring-boot-sample-batch</module>
 		<module>spring-boot-sample-data-elasticsearch</module>
 		<module>spring-boot-sample-data-gemfire</module>