Add service interface to camshiftdemo

add service interface: `~set_roi` with `sensor_msgs/SetCameraInfo`, not only selection by mouse clicking.

sample code for the client.

```python
#!/usr/bin/env python

import rospy
from sensor_msgs.msg import *
from sensor_msgs.srv import *

rospy.init_node("hogehogehgoe")


s = rospy.ServiceProxy("/camshiftdemo/set_roi", SetCameraInfo)
cam_info = CameraInfo()
cam_info.roi.x_offset = 0
cam_info.roi.y_offset = 0
cam_info.roi.width = 100
cam_info.roi.height = 100

s(cam_info)
```