Towards interactive version with a dist sensor
General framework for dist sensor in a short script here
* Use speed of sound and max dist to calculate cutoff time needed to
  avoid flicker
* Refractor code to a brightness function
Start of an interactive cube version via a distance sensor
Better comments on distance sensor
Spec says 20uS wait needed, so reduce 1000 to 100uS in our case.
better comment
Merge branch 'master' of github.com:ingegno/fecube