prevent the system from sleeping when a user is looking at the screen.

this is accomplished using the built-in camera on my laptop screen's top bezel
and opencv to detect a face.  i'm going to use the API used by the default movie
player in ubuntu for preventing system sleeps.

OTHER IDEAS:
* detecting a face could simulate user activity.  this would prevent sleeping
and bring up the password prompt when the system is already asleep.
	* how cool would that be?  if the system is asleep and you sit down at it, the
	password prompt comes up before you touch anything!
	* or if the screen saver doesn't lock the screen, then the laptop just wakes
	up.
	* DONE!
* fire up the face detector only just before going to inactive mode.
	* that way the camera isn't running all the time
	* maybe start 10 seconds before going inactive, then run until activity
* authenticating a user based on face recognition (as opposed to face
detection)
	* much more difficult to do reliably
* be more aggressive with turning off the screen.  dim the screen as soon as
it's not being looked at.  turn it off soon thereafter.
	* could be enough to save battery.  would need to power the camera sometimes,
	but would save on backlight.  need to test.
* don't prevent sleep when face is detected, but user isn't actually looking at
screen.
	* requires pose estimation.
	* also requires estimating the direction of the eye.