Set default logging to WARN for Spark streaming examples.

This programatically sets the log level to WARN by default for streaming
tests. If the user has already specified a log4j.properties file,
the user's file will take precedence over this default.
Minor clean-up
Merge pull request #363 from pwendell/streaming-logs

Set default logging to WARN for Spark streaming examples.

This programatically sets the log level to WARN by default for streaming
tests. If the user has already specified a log4j.properties file,
the user's file will take precedence over this default.