trim '/' prefix of topic and service to change to relative name-space

Before this PR is not patched, we cannot change namespace of camera topic and service. For example, 

~~~ sh
% ROS_NAMESPACE=stereo roslaunch ueye_cam rgb8.launch
~~~

~~~ sh
% rostopic list
/camera/camera_info
/camera/image_raw
/camera/image_raw/compressed
/camera/image_raw/compressed/parameter_descriptions
/camera/image_raw/compressed/parameter_updates
/camera/image_raw/compressedDepth
/camera/image_raw/compressedDepth/parameter_descriptions
/camera/image_raw/compressedDepth/parameter_updates
/camera/image_raw/theora
/camera/image_raw/theora/parameter_descriptions
/camera/image_raw/theora/parameter_updates
/rosout
/rosout_agg
/stereo/nodelet_manager/bond
/stereo/ueye_cam_nodelet/parameter_descriptions
/stereo/ueye_cam_nodelet/parameter_updates
~~~

After patched, we can do it.

~~~ sh
% rostopic list
/rosout
/rosout_agg
/stereo/camera/camera_info
/stereo/camera/image_raw
/stereo/camera/image_raw/compressed
/stereo/camera/image_raw/compressed/parameter_descriptions
/stereo/camera/image_raw/compressed/parameter_updates
/stereo/camera/image_raw/compressedDepth
/stereo/camera/image_raw/compressedDepth/parameter_descriptions
/stereo/camera/image_raw/compressedDepth/parameter_updates
/stereo/camera/image_raw/theora
/stereo/camera/image_raw/theora/parameter_descriptions
/stereo/camera/image_raw/theora/parameter_updates
/stereo/nodelet_manager/bond
/stereo/ueye_cam_nodelet/parameter_descriptions
/stereo/ueye_cam_nodelet/parameter_updates
~~~