Add SFL4J logger with Log4J backend.

* Add SFL4J abstract logger with Log4J as backend implementation.
* Add log4j properties configuration for each environment.
Merge branch 'master' into add-log4j

Conflicts:
	src/main/java/org/jcaching/server/ClientConnection.java
Merge pull request #6 from jmg/add-log4j

Add sfl4j with log4j