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.