Clearly #define eeprom as optional, default disabled.
255 seems to give little added brightness over 100 or 128 but much
highger current consumption.  So it makes sense to use a lower
default value and use 255 perhaps only riding in plain daylight.
TODO: make it runtime configurable with "brake gestures" maybe.
Fix the direction of a less than operator.
Better gyro speed runtime calibration, decrease correction rates.

Now compares the rotation rate given by the gyro against that calculated
naively from the accelerometer readings, not relying on the gyro
zero position and the current angle to be correct.  Also slash all
three correction factors by 2 as the calibration should now work much
better and not be needed that much after the initial catch-up.  This
will decrease later effects of braking etc. on the gyro-based angle.

Also change the gyro rate factor default, for some reason the documented
value seems to have been about half of what's really needed.