Switch to SLF4J

There are 2 compelling reasons to switch from log4j to slf4j:

• HBase provides a client library that is going to be embedded in another application. Using SLF4J lets the application chose whatever logging library it wants instead of imposing log4j.
• When using SLF4J, we should use logback by default as it is basically a better, faster, stronger log4j. Same author, new design / new code. See http://logback.qos.ch/reasonsToSwitch.html