Add logback sample application

Add a logback sample application to demonstrate how `logback-spring.xml`
can be used for customization.

See gh-2558

spring-boot-samples/pom.xml

@@ -55,6 +55,7 @@
 		<module>spring-boot-sample-jta-bitronix</module>
 		<module>spring-boot-sample-jta-jndi</module>
 		<module>spring-boot-sample-liquibase</module>
+		<module>spring-boot-sample-logback</module>
 		<module>spring-boot-sample-metrics-dropwizard</module>
 		<module>spring-boot-sample-metrics-opentsdb</module>
 		<module>spring-boot-sample-metrics-redis</module>