Partial Update support

This is an Alpha version of partial update support
(send the 'P' character to "command")

The demos CounterDemo.py and PartialDemo.py tryout this feature

Note that after several cycle of partial update there may be some
contrast degradation, so occasionally send a full update; Just sending
the 'U' command (or the update method in EPD.py) will fully refresh
the current image.

Signed-off-by: Christopher Hall <christopherhall.hsw@gmail.com>