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/spring-boot-sample-logback/src/main/java/sample/logback/SampleLogbackApplication.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2012-2015 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package sample.logback;
+
+import javax.annotation.PostConstruct;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+@SpringBootApplication
+public class SampleLogbackApplication {
+
+	private static final Logger logger = LoggerFactory
+			.getLogger(SampleLogbackApplication.class);
+
+	@PostConstruct
+	public void logSomething() {
+		logger.debug("Sample Debug Message");
+		logger.trace("Sample Trace Message");
+	}
+
+	public static void main(String[] args) throws Exception {
+		SpringApplication.run(SampleLogbackApplication.class, args);
+	}
+
+}