Minor Simulator Refactors

[rameshvarun]

Some minor simulator refactoring. These changes are made on top of #14.

1. I turned Piece into an enum. There was no reason why it was a full class, since the only property of each instance was an enum anyway.
2. I changed the winner from being a nullable player into an enum. This lets us represent four disjoint states: the winner is undetermined, the attacker has won, the defender has won, and the game ended in a tie.

[jlwatson]

delete branch?

[rameshvarun]

Yeah - typically, unless you want to add more stuff to the same 'feature', you can delete the branch.