Interfacing camera control registers with services.

Possible implementation (draft) of https://github.com/ros-drivers/camera1394/issues/30.

Other dc1394*get/set*register(s) functions could be wrapped in a similar way. The messages are generic to be used with all of them.

I tested it on Ubuntu 12.04/Hydro with Ladybug 3, selecting/querying sensors (side cameras, top camera) participating in auto exposure using
```
rosservice call /camera/set_camera_registers "{type: 0, offset: 0x1E90, value: [0b111110]}"
rosservice call /camera/set_camera_registers "{type: 0, offset: 0x1E90, value: [0b000001]}"
rosservice call /camera/get_camera_registers "{type: 0, offset: 0x1E90, num_regs: 1}"
```
and selecting/querying images for transmission (all cameras, side cameras).
```
rosservice call /camera/set_camera_registers "{type: 0, offset: 0x1E88, value: [0xffffff]}"
rosservice call /camera/set_camera_registers "{type: 0, offset: 0x1E88, value: [0xfffff0]}"
rosservice call /camera/get_camera_registers "{type: 0, offset: 0x1E88, num_regs: 1}"
```