pulling the human-move interaction out of play() function
removing unneded resetting of variable in dispboard()
1) changed encoding of "random move" from int(7) to None, and added function to calculate a ranadom move right in the minimax alg, rather than relying on the calling function to know that 7 means pick a random one for me.
2) rather than incrementing and decrementing depth w/in the minimax algorithm, do the increment w/in the recursive function call.
3) I realized that minimax neatly cleans up the board state after itself, so no need to do any type of deep copy of input boards
whoops - cleaning up board variables in minimax that should have been part of last commit
updated minimax docstring