Update signaling to match spec. Fixes #3.

[Andy4495]

Update control signals to better match US2066 driver chip:

• Only enable SCK and MOSI pins when display is being sent control/data
• Reverse SCK logic to toggle low-high-low instead of high-ow-high

Update example so that the single-character-update portion is easier to monitor by clearing display first and adding a small delay between character updates.