Move rollback logic to chessboard
Show message when saving game in GUI
Use lighter rollback logic

The new rollback logic resets the board and then performs all the moves up to
that point again. This way only a string history needs to be stored.

The history is stored as string tuples: Start and goal fields.
Move saving is now done with string tuples

Fixes #7. I will have to see how much this breaks client code. My GUI works with it.
Update README