SPARK-1190: Do not initialize log4j if slf4j log4j backend is not being used

Author: Patrick Wendell <pwendell@gmail.com>

Closes #107 from pwendell/logging and squashes the following commits:

be21c11 [Patrick Wendell] Logging fix
(cherry picked from commit e59a3b6c415b95e8137f5a154716b12653a8aed0)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>