Added screensaver with default 1 minute turn on.

[rickyphewitt]

Fixes #20

This change adds a task that runs every minute (hopefully configurable soon) and turns the screen off if no key press was registered in that time. When you press a key (no rotary/gesture support yet!) the screen will wake up.

There are various areas I'd like to improve, but this should help keep any burn in to a minimum :).

Initially I was looking at adding an animation to the screen but everything I read about OLED's said that a screensaver like that does not prevent burn in.