Improve comments on some register values and cleanup unused/commented stuff.
Added safety feature to disable motor if i2c master is gone.

When motor is ON (Speed register not 0 nor 0x80), the i2c
master must re-write the register at least once every 255ms
(nominal), otherwise the bridge controller will reset,
freewheeling the motor.
Optimize the bridge controller pins initialization to reduce code size.
Bridge Ctrller sw: Added dissipative break mode (untested).

The command register can be written with 0x81 to enable dissipative
breaking (using the lower FETs); if motor spinning, current speed is
saved.
If written with 0x80, dissipative breaking is turned off and saved speed
is set.

Reading the command register will return 0x80/0x81 if one of these
commands has been previously written and no other commands were written
meanwhile.
Merge branch 'MarkIV_RasPi_NJAY' of https://github.com/vnevoa/DiffTrike into MarkIV_RasPi_NJAY