[FLINK-1086] Replaced jcl with slf4j and log4j with logback.

Replaced the jcl logging interface with slf4j and log4j with logback. Removed also log4j specific code so Flink can easily be executed using a different slf4j binding. This work also standardises the logging configuration which is now exclusively done by specifying a logback.xml configuration file. This file is either put in the classpath (e.g. src/main/resources/) or given via the environment property -Dlogback.configurationFile. If no configuration is provided then the default configuration of logback is used.
Nice that you didn't forget to update this :+1:

I'm not sure if the Copyright line is necessary, because you have it in the lower section. And for the name, I think we should have it as LOGBack.