Undo feature

Great job with the app Daylen!

I've added in some basic undo functionality. Pressing Ctrl-Z (or selecting Edit->Undo) removes the latest move and resets the board to reflect the penultimate position. I've also removed a few items that don't need to be checked in, and turned off code signing for development builds (let me know if this is a problem).

On an unrelated note, I haven't been able to analyze a position with the Stockfish engine without the program crashing (even when building from a fresh clone of the repo). Is this related to the code signing that you have in place? The engine analysis works when running the App Store version.