=Griffeath's cellulare automaton implementation

Author:: Michel Belleville <mailto:michel.belleville@gmail.com>

Rules :
* cells may evolve cycling through (n) states (4 by default).
* first state (0) is a "null" state by convention.
* a cell evolve to the next state when at least 3 of its neighbors are already
  on said next state.
* a cell that doesn't evolve remains on the same state.