Try a better brake detection logic.

First of all fix the "low pass filter" to be same scale as the variable
and then shift left by 4 bits to add some precision.  Secondly only
compare its value against another LPF (an even slower one) to detect
momentary braking vs. just slowing down.  We still use absolute values
in the comparison, may want to make them depend on the recent
variability (std deviation) or perhaps make the min detectable brake
period depend on that.  We also still need to take timediff into account.

Update README.