Merge pull request #11 from oTree-org/lemon-market

Lemon market
refactoring
.
Volunteer's dilemma
Merge pull request #12 from oTree-org/volunteer_dilemma

Volunteer's dilemma
Bertrand Competition
Merge pull request #13 from oTree-org/bertrand_competition

Bertrand Competition
Update README.md
lemon_market: use random period payoff