instead of commons-logging depend on slf4j-api

Instead of the commons-logging framework depend on slf4j. JCL discovery process relies on classloader hacks to find the logging framework at runtime but this mechanism leads to numerous problems including unexpected behavior, hard to debug classloading problems resulting in increased complexity. This is nicely captured by Ceki (the author of Log4J, SLF4J and Logback) in http://articles.qos.ch/thinkAgain.html