2-bpp partial update

This release adds unique functionality - the ability to use partial updates (non-flashing transitions) in 2-bpp grayscale mode. Small changes have also been made to the fullUpdate() method:

• If CLEAR_NONE is chosen, the 1 and 2-bpp modes will push all pixels towards white and black simultaneously. This will produce correct looking output from any starting point, but does not respect the balance of charge. Use this feature carefully because repeated pushes in a single direction can lead to temporary charge imbalance and a "graying" effect.
• If some form of clearing is done, the update will assume that the display already white and only push black/gray pixels.