Chdk power off

This pull request is to allow chdk cameras to be switched off by changing the pulse width for Canon cameras. One possibility is to use 250ms for a single pulse, 750ms for shutting the camera off.  this script has been tested on an IXUS 220 HS and is working by clicking the buttons on the gcs console. Camera shuts down and pictures are taken correctly.

I'm still working out some final details on the chdk script, but don't see a good location to put the file in the paparazzi repository. I was expecting something like a scripts or external directory for that. 

Also note that the exact timing for actuating the shutter is off by 250ms (one quartersec), at least for the led_cam_ctrl. The DC module is actuated at the end of the 4hz function in the led_cam_ctrl.c file. The shutter LED is turned on/off already. So in total it takes 2 4Hz passes to shut down the LED again, which is 500ms. For me, I'm just setting the shutter_delay to 0, because I don't fully know the effects of bringing this DC function call at the top of the function.