Add org.hibernate.SQL to the --debug logger levels

I think this fixes gh-446 (it seems unreasonable to want to log the DDL
even if it is not being executed)

spring-boot-samples/spring-boot-sample-data-jpa/src/main/resources/application.properties

@@ -0,0 +1 @@
+debug=

spring-boot/src/main/java/org/springframework/boot/logging/LoggingApplicationListener.java

@@ -91,6 +91,7 @@ public class LoggingApplicationListener implements SmartApplicationListener {
 		LOG_LEVEL_LOGGERS.add(LogLevel.TRACE, "org.apache.catalina");
 		LOG_LEVEL_LOGGERS.add(LogLevel.TRACE, "org.eclipse.jetty");
 		LOG_LEVEL_LOGGERS.add(LogLevel.TRACE, "org.hibernate.tool.hbm2ddl");
+		LOG_LEVEL_LOGGERS.add(LogLevel.DEBUG, "org.hibernate.SQL");
 	}
 
 	private static Class<?>[] EVENT_TYPES = { ApplicationStartedEvent.class,