#11 brightness coding of success

[Allen Lee]

The goal here is to use a brightness coding to simulate
accumulation of success. When a player runs over a
piece of food their dot becomes brighter, and more
shiny to other players.

Implementation notes:
- brightness fades over time.
- either linearly increase/decrease brightness or do it
as a percentage (or a combination of both)
- leaky integrator model or integration with a time
bias, so we're communicating a measure of the success
of participant to the other participants. Their
success is a function of how much food they've received
in addition to a time component modifier (if they got a
lot of food earlier but haven't gotten any recently
they suck).
- configuration parameters to turn on/off (also
parameterize whatever function we decide to use to
model the increase/decay of brightness).