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.
It was different output I guessed, like that
~~~
[DEBUG] [1417617965.482413574]: Connection::drop(2)
[DEBUG] [1417617965.482618652]: TCP socket [11] closed
[DEBUG] [1417617965.482651088]: Connection::drop(0)
[DEBUG] [1417617965.482689186]: Connection::drop(2)
[DEBUG] [1417617965.482716707]: Connection::drop(2)
[DEBUG] [1417617976.136166330]: Accepted connection on socket [8], new socket [11]
[DEBUG] [1417617976.136255934]: Adding tcp socket [11] to pollset
[DEBUG] [1417617976.136298063]: TCPROS received a connection from [127.0.0.1:52177]
[DEBUG] [1417617976.136412683]: Connection: Creating ServiceClientLink for service [/moge_1417617868470966225/set_logger_level] connecte\
d to [callerid=[/rosservice] address=[TCPROS connection to [127.0.0.1:52177 on socket 11]]]
[DEBUG] [1417617976.136458160]: Service client [/rosservice] wants service [/moge_1417617868470966225/set_logger_level] with md5sum [*]
[DEBUG] [1417617976.137811498]: Socket [11] received 0/4 bytes, closing
[DEBUG] [1417617976.137931906]: TCP socket [11] closed
[DEBUG] [1417617976.137968672]: Connection::drop(0)
[DEBUG] [1417617976.138010621]: Connection::drop(2)
[DEBUG] [1417617976.138060102]: Connection::drop(2)
[DEBUG] [1417617976.265935701]: Accepted connection on socket [8], new socket [11]
[DEBUG] [1417617976.266018264]: Adding tcp socket [11] to pollset
[DEBUG] [1417617976.266042794]: TCPROS received a connection from [127.0.0.1:52184]
[DEBUG] [1417617976.266093908]: Connection: Creating ServiceClientLink for service [/moge_1417617868470966225/set_logger_level] connecte\
d to [callerid=[/rosservice_25142_1417617976137] address=[TCPROS connection to [127.0.0.1:52184 on socket 11]]]
[DEBUG] [1417617976.266118428]: Service client [/rosservice_25142_1417617976137] wants service [/moge_1417617868470966225/set_logger_lev\
el] with md5sum [51da076440d78ca1684d36c868df61ea]
[DEBUG] [1417617976.266410200]: Connection::drop(2)
[DEBUG] [1417617976.266502428]: TCP socket [11] closed
[DEBUG] [1417617976.266539304]: Connection::drop(0)
[DEBUG] [1417617976.266567819]: Connection::drop(2)
[DEBUG] [1417617976.266595102]: Connection::drop(2)
~~~