[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.
I'm not sure either. Maybe we should ask our License/Dependency guru @StephanEwen what's the right way to do it.