Adds findCellValue method to game prototype.
Removes boardSize property from game objects.
In findCellValue, relatedValues now stores digit values, not indeces.
findCellValue only assigns one new value before returning.

Previously, any solvable cell got assigned its value and every higher
one, so they all came out "9".
Adds checkAllCell and solve methods.
Merge pull request #2 from ambirdsall/solve_deterministically

Solve deterministically