add opencv_apps, proposed in #40

- package names opencv_apps? opencv_examples?
- this package contains a lot of new message, show we create opencv_msgs that holds opencv basic data structure ? or ask other packages (geometry_msgs or others ) to add new messages?

here is how to run program, if you add `_debug_view` you can see debug view

`rosrun opencv_apps lk_flow _debug_view:=true image:=/image_raw`