Add listener to persist relationship between job and task

This introduces a listener that stores the association between a Spring
Batch job and the task it was executed within.

Resolves spring-cloud/spring-cloud-task#46

Merge Changes based on code review.

spring-cloud-task-samples/timestamp/src/main/java/org/springframework/cloud/task/timestamp/TaskApplication.java

@@ -40,6 +40,8 @@ import org.springframework.context.annotation.Bean;
 @EnableConfigurationProperties({ TimestampTaskProperties.class })
 public class TaskApplication {
 
+	private static final Log logger = LogFactory.getLog(TaskApplication.class);
+
 	@Bean
 	public TimestampTask timeStampTask() {
 		return new TimestampTask();
@@ -53,7 +55,6 @@ public class TaskApplication {
 	 * A commandline runner that prints a timestamp.
 	 */
 	public class TimestampTask implements CommandLineRunner {
-		private final Log logger = LogFactory.getLog(TimestampTask.class);
 
 		@Autowired
 		private TimestampTaskProperties config;