add set_logger_level func

for jsk-ros-pkg/jsk_common#106

If this is merged, you can set logger_level with (ros::set-logger-level 3)

I tried to implement  set_logger_level for implementing function like ``rospy.init_node("wait_test2", log_level=rospy.DEBUG) ``.
But I am not sure doing this is right way to implement it.
name of logger "ros" will be logged all ros log message. Is it OK?
"ros.roseus" will be logged message from roseus.
Or, logger name can be set by ROSEUS_SET_LOGGER_LEVEL.