Luhn algorithm

[dumindarw]

I would like to add Luhn algorithm(https://en.wikipedia.org/wiki/Luhn_algorithm) to "src/algorithms/uncategorized/" is it ok for you?

[leeoniya]

here's a cute one, FYI: https://gist.github.com/ShirtlessKirk/2134376

[bigtone1284]

@trekhleb @dumindarw @leeoniya

#368

I added this PR in response to this open issue. Please review it when you can! Thanks.