Convert Mondrian to use SLF4J.

SLF4J is a more appropriate choice for a library that can be embedded
within other systems. Additionally it solves MONDRIAN-1171, where
Mondrian is relying on a log4j private API that is not present in
slf4j-log4j bridges.

TODO:
* CmdRunner relies on manipulating an actual log implementation. I'm
not sure this should even be present in the mondrian JAR file, or if
it should be in a supporting module.

* The tests rely on manipulating logging, and so require a concrete
implementation. I have used logback for this, but I've hacked it together.
I can't run the tests locally so I can't tell if more work is needed here.

Signed-off-by: Nigel Magnay <nigel.magnay@gmail.com>