1037 country guessing

@Kartones can you please review?
Actually:
a) if sample.count == 0 the matches will be == 0 and 0.0 / 0 will return NaN
b) if countries.count == 0 then matches will be 0

and NaN cannot be true compared to any number (not even with NaN). So it is safe, but I can handle the ZeroDivisionError for clarity's sake and your peace of mind :)

I'll add tests for the zero cases.