Add while loop to keep game alive until all solved

* Created a zeroChecker for the Guesser
* Created a while loop that would keep the game going until
  all of the zeros have been changed to a value
* Add logic to guessOnCell to tell the cell to change its value
  when only one value is possible
* Add logic to Cell to change its value when told
* Add some tests

Next steps:
* Make the code look a little less ugly. Separate the concerns
  and try to get away from having to repeat a for loop ALL the time