Create internal representation of the game grid

[lindboe]

It's fine to start this off naive, but I would like to later take a look at adopting the state handling patterns used in Alex Kehayias's talk: https://www.youtube.com/watch?v=TW1ie0pIO_E

We'll want it to indicate which squares are occupied and eventually by which type of thing (enemy, player-controlled character maybe to start?)