Add clustered distribution
Refactor simulation to get tit-for-tat dominating
Merge remote-tracking branch 'upstream/master' into dmd-tinker

Conflicts:
	assets/browser.js
	browser.coffee.md
	simulation.coffee.md
Added stag hunt and snow drift.

Added functions that provide the Stag Hunt and Snow Drift variants of
the Prisoner’s Dilemma.