Changed default sketch display

Removed Arduino logo graphic and set background color to black to save
power
Added 8 pixel border padding to text to improve visibility through bezel
at large viewing angles
Added basic low-power stop mode control

Added ability to configure external interrupt (EXTI) pins as wakeup
event sources for sleep/stop mode via CPU::setWakeSource.
CPU::setLowPowerMode configures STOP mode to keep as little as possible
on besides the RTC.
Added clearEvents() and waitForEvent() functions to CPU for entering
stop mode cleanly.
Added CPU::reboot() to wrap reset functionality