A card game invented by Dave Turner, inspired by the chapter in Jared 
Diamond's _Collapse_ where the Vikings go to Greenland and then they all die. 

Greenland is currently playable in the terminal - just navigate to the directory 
where you saved it and type in "ruby greenland-play.rb" and have fun!

To run simulations with pre-set strategies, run "ruby greenland-sim.rb" instead.
(Though the only strategies currently defined are the human-user StdInput, and
the deadly default, DoNothing.)

Next steps:

1. Define a few other strategy subclasses.

2. Run simulations with those strategies to test gameplay and see if any of them 
actually work, or if the game rules need to be modified.