Add a logging system, starter and sample for Log4j 2

Closes gh-1565

spring-boot-starters/pom.xml

@@ -41,6 +41,7 @@
 		<module>spring-boot-starter-jta-bitronix</module>
 		<module>spring-boot-starter-logging</module>
 		<module>spring-boot-starter-log4j</module>
+		<module>spring-boot-starter-log4j2</module>
 		<module>spring-boot-starter-mobile</module>
 		<module>spring-boot-starter-actuator</module>
 		<module>spring-boot-starter-parent</module>